A NEW SPECIES OF THE GENUS CRENILABIUM ( MOLLUSCA , HETEROBRANCHIA , ACTEONIDAE ) FROM BRAZIL

Crenilabium birmani, a new species of acteonid is conchologically described from the southwestern and southern coast of Brazil. The new species is the fist record of the genus for the South Western Atlantic and differ in details and proportions of the suture and aperture. Two variants of the suture were detected, some specimens have simple suture, while others possess a deep, scaled suture. Information about a possible specimen of Crenilabium figured by Abbott (1974) representing Rictaxis punctocaelatus is here discussed, and its holotype if figured.


INTRODUCTION
The acteonids are small gastropods that normally live in infratidal environment.The opisthobranch organization of their bodies contrasts with the heavy and well-developed shells.The genus Crenilabium Cossmann, 1889 [type species Actaeon (C.) aciculatus Cossmann, 1889:307 (pl. 8, fig. 30), OD, a Tertiary fossil from Europe] is sometimes considered subgenus of Actaeon Montfort, 1810 (e.g., Thiele, 1992:629).The genus encompasses species with elongated shell, few transversal lines at columellar margin of the shell aperture, and absence of fold at inner lip.The genus was never reported to Brazilian waters (Marcus, 1972;Rios, 1994).
In dredges off the southwestern and southern Brazilian coast, some shells belonging to Crenilabium were collected, revealing a new species formally described here.

Etymology:
The specific epithet is in honor of the discoverer of the species, the physician and conchologist Adolpho Birman, from São Paulo, SP.Brazil.

DISCUSSION
There is variability in some characters of Crenilabium birmani.One of them is the dissimilar shape of the suture, in which some specimens have a simple and shallow furrow (Figs. 1, 2, 3), while it is deep, scaled in others (Figs.4-9).The form of the aperture is also somewhat variable; the deepest portion of the inner lip is almost in middle level in some specimens (Fig. 1), while it is between middle and inferior thirds in others (Figs. 5, 7).The inferior or siphonal region of the aperture can be also different, it is rounded sometimes (Fig. 1, 6, 7), while it is somewhat squared in others (Fig. 5), having a clear angle between inner lip and siphonal edge.There is a coincidence between the two morphs described above, represented each one in the Figures 1 and 5 that induced the initial interpretation of two species.The morph represented in the Fig. 1 is found in shallower waters (53-55 m depth); while the other, represented in the Figs. 5 and 6, with scaled suture, is found deeper, at about 200 m.Despite a conservative approach is given here, as both morphs considered a single species, further studies can change this scenario.However, the suggestion is that both morphs are actually because of adaptations to different deepness.Related to the geographic distribution (Fig. 12), specimens with simpler suture (Fig. 1) are for moment only know from Rio de Janeiro to north São Paulo coast; in the mean time, specimens with scaled suture occur from Rio de Janeiro to the south coast of Rio Grande do Sul.
The placement in the genus is based mainly on the elongated shell shape, the relatively thick shell walls, and by the absence of folds at the inner lip.These characters are present in the species described herein that approach it from the type species Crenilabium aciculatum (see Zilch, 1959Zilch, -1960:8, fig. 13:8, fig. 13).The fold in the inner lip is present in other elongated shelled genera of acteonids, such as Tenuiacteon Aldrich, 1921, and Actaeonidea Gabb, 1873 (Zilch, 1959(Zilch, -1960:8-9):8-9), from which the presently described species can not belong, as it lacks this fold.
The described species can not be confused with any other Crenilabium species, however, it is similar to the Mediterranean and North Atlantic C. exile (Jeffreys, 1870) [= Acteon nitidus Verrill, 1882:540, pl. 58, fig. 21], from which C. birmani differs in having proportionally longer aperture, spire with deeper suture and last whorl proportionally longer.Crenilabium birmani additionally differs from the type species of the genus, C. aciculatum, by deeper suture, by shorter proportion of the aperture and by wider and less rounded shape of the anterior region of the aperture.
A specimen surprisingly similar to Crenilabium birmani, and to Crenilabium exile, is figured by Abbott (1974:312, fig. 3906) to represent the Californian Rictaxis punctocaelatus (Carpenter, 1864).On the other hand, the examination of the holotype of R. punctocaelatus (Figs. 10,11) revealed completely different characters.Most subsequent pictures of that species never showed a different shape of the holotype [e.g., Marcus, 1972 (fig. 5); Rudman, 2001], which allow that Abbott's picture may be a misidentification of a Crenilabium species, possibly to the C. exile (species number 3908 of that catalogue).The species described here differ from that Abbott's specimen in having a proportional longer aperture and by less straight spire.