-62 PROCEEDINGS OF THE ACADEMY OF 



GASTEEOPODA. 



Genus PLATYCERAS, Conrad, 1S40. 



(Acroculia, Phillips, 1841.) 



The germs PI at tic eras was proposed hy Mr. Conrad for a group of palreozoic 

 shells, Tery generally referred by European authors to the Montfort's genus 

 Ca'pulu*, published in l&10,=(Pileopsis, Lamarck, 1812.) Mr. Conrad's de- 

 scription of this g^nus reads as follows : " I propose to group in this genus 

 the PileOpsi.s tubifer, (Sowerby), P. rehtsa, (Sowerby), Nerita haliotis, 

 (Sowerby), and perhaps Bellerophon eornuarietes. These shells are suboval or 

 subglobose, with a small spire, the whorls of which are sometimes free and 

 soniet mes contiguous ; the mouth is generally campanulate or expanded."* 

 During the following year, Prof. Phillips proposed in his "Palreozoic Fossils," 

 p. 93, the name Acroculia for the same fossils. 



In this country Mr. Conrad's name has been generally adopted for these shells, 

 which is certainly proper, unh-ss they shall be found to agree with the older ge- 

 nus Capulus, since his name has priority over that proposed by Prof. Phillips. 

 Although agreeing with those who regard these fossils as being probably dis 

 tinct from the existing genus Capulus, we believe they are more nearly allied 

 to that group than is generally supposed to be the case by American palaeon- 

 tologists. The only reason assigned by Professor Hall for separating them 

 from the modern genus is, that he had never observed in them any traces of 

 the peculiar horse- shoe shaped muscular scar so conspicuous in the genus 

 Capulus.^ We have recently, however, found very similar muscular impres- 

 sions in two distinct species of this genus, one of which seems to be a variety 

 of P. subrectum, Hall, from the Keokuk group, while the other is a new 

 species describ <1 in this paper from the Waverly Sandstone, of Ohio. J In 

 both of these, internal casts show an elongate oval muscular impression on 

 each side, connected by a linear band passing around behind. It is also 

 worthy of ii"te that both of these species belong to the nearly or quite straight 

 section of the genus, for which Prof. Hall at one time proposed the name 

 of Orth onychia, and hence are less nearly like the modern typical forms of 

 the genus Caputus than the great majority of the Pala?ozoic species. 



A careful examination of extensive collections of these shells from our west- 

 ern pala?ozoic rocks, has also satisfied us that the animal must have been simi- 

 lar in habit- to Capulus and other types of the family Capulidce, to which they 

 evidently belong||, in being sedentary shells. This is shown by specimens 

 found attached to crinoids and other objects in such a manner that the 

 sinuosities of the lip exactly correspond to the irregularities of the surface to 

 which they are attached. For instance, we have now before us one of these 

 shells attached to the side of a Peulreinitrs Godoni, so as to entirely cover one 

 of the pseudo ambulaeral fields and two of the intermediate areas, and yet the 

 sinuosities of its lip conform so exactly to the irregularities of the side of the 



* Palaeontolugical Keport, New York, 1840, p. 205. 



f 12th Ann. Keport Kegents Uiiiversi y New York, p. 16. 1S59. 



\ Similar muscular impressions are km>wn to occur in the Neritidx and other univalves. 



\ It-port 4th I)ist. N. Y., 1843. 



1 1 In a sheet entitled " Iowa Geological Survey, supplement to vol. 1. part ii, 1S5SV" issued in 

 1860 Prof. Hall described a patelliform Platyceras, lr> in Nauvoo, Illinois, under the name /'.//.- 

 sureUa, which In- says lias a perforation just anterior to the apex. Although this is merely men- 

 tioned as a specific character, distinguishing n from an otherwise similar species described in the 

 same paper, conchologists will readily understand that such an opening:, near the apex of the 

 shell, if oat oral must have been, judging from all analogy, for an excurrent or anal siph >n , as 

 in the FissureUidee, and hence would not only remove the species from tbegenus Plalyctras. hut 

 from the family Captdidie, and place it in the Fissurellidse, regarded hy the best systtmatists as 

 belonging t i a distinct order fri m that including the Capulida . A careful examination, however, 

 nfthe typical specimens ol P. fissurella, and other examples of the same species from the original 

 locality, now in ihe p sses-ion of one of ihe writers, leads us t" think the perforation alluded to 

 (whii h only exists in nne of the specimens), almost hejond doubt an accidental break in the shell, 

 not a natural perforation 



[July, 



