No. 186.] 15 



OBSERVATIONS 



ON THE GENERA 



CAPULUS, PILEOPSIS, ACROCULIA, AND PLATYCERAS. 



In the work entitled "Figures and Descriptions of the Palaeozoic 

 Fossils of Cornwall, Devon and West-Somerset," by Professor Phillips, 

 published in 1841, he has proposed the Genus Acroculia to include 

 certain fossils which had been referred to the Genus Pileopsis, and to 

 which, he remarks, they "offer but slight analogy." His description is 

 as follows : 



" Provisional character. Obliquely spiral ; the apex free, the aperture 

 "ample, without columella: a sinus in the right lip." 



This generic distinction has been acknowledged by some palaeontolo- 

 gists ; but, more recently, both Continental and English naturalists have 

 referred all these forms to Capulus or Pileopsis, which are regarded 

 as synonymous. 



Previous to the publication of the work of Professor Phillips cited 

 above, Mr. Conrad, in his Report on the Palaeontology of New-York 

 for 1840 (p. 205), proposed the generic name Platyceras, with the 

 following remarks : 



" I propose to group in this genus the Pileopsis tubifer ( Sowerbt ), P. 

 " vetusta ( Sowerby), the Nerita haleotis (Sowerby), and perhaps Bslh- 

 " rophon cornuarietes. The shells are suboval or subglobose, with a small 

 * spire, the whorls of which are sometimes free and sometimes contiguous : 

 " the mouth is generally campanulate or expanded. I have not seen a species 

 " above the Silurian rocks, though they probabl)^ occur above them in Europe*, 

 "and they are never found in the Lower Silurian strata ; they characterize 

 " the middle portion of the system." 



The generic description of Mr. Conrad is more comprehensive than 

 that of Professor Phillips, as it includes shells with the volutions free 

 or contiguous. Both authors, however, have designated among the typi- 

 cal forms the Pileopsis vetusta of Sowerby. The species first described 

 by Mr. Conrad are the P. dumosum of the Upper Helderberg lime- 

 stone, the P. ventricosiim and P. gebhardiiirom the Lower Helderberg. 



The P. dumosum is spiniferous, and the volutions in all these are essen- 

 tially contiguous ; but some species subsequently referred hj Mr. Conrad to 



♦ See note on page 9. 



