62 G. LINDSTRÖM, UN TllE HILUKIAN GASTKOPODA AND TTEKOPODA OF GOTLAND. 



seqiience of what now has been adduced I cannot but consider Strophostylus a]«o as 

 idcntical with Platyceras. 



Besides, the highly variable and proteic forms of this genus have been ranged 

 within niany other genera of which sorne are enumerated here below. 



Exogyroceras a subgeniis proposed by Meek and Wohtuen in 1868, Geol. Survey 

 uf Illinois vol. III p. 509 for shells with a sinistral spire and obscure coluraella, 



Igoceras Hall. Established by Hall in 1859 in Pal. of New- York, vol. III p. 

 330. It ditfers from the straight Platycei^ata (Orthonychia) only in having the sui'face 

 cancellated and it can consequently not be retained on so trifling a distinction. 



Natica Adanson. Both Pictet and D'OiiBiGNY think that this genus occurs in 

 the Silurian formation, but such species as Natica parva, Natica spirata or Wenlocken- 

 sis (Sow. Sil. System pl. 5 & 12 and I)'Orbigny Prodr. I p. 29) are true Platycerata. 

 It is possible that also some of the Bohemian Natica^, N. gregaria for instance, belong 

 to this group. 



Nerita L. is employed by Sowerby in the Silurian System for the most common 

 Platyceras. 



• Orthonychia Hall Rept. 4th distr. N.-York Geol. Survey p. 172 is a subgenus, 

 created for the straight Platycerata, forming an elongate cone, with small spire and 

 the ultime part very large. In the 12th Report N.-York State Cab. p. 18 Hall de- 

 clares that a renewed examination has not showu any reliable characters, by which 

 they may be separated from Platyceras. Meek and Wurthen, however, retain this name 

 and. it might perhaps be advisable to do so for all those species which are invariably 

 straight or scalaroid. 



Then there may be some few species which belong to this group, but have been 

 placed with the genera Holopea, Isonema, Naticopsis. Professor Ferd. Roemer placed 

 one of the forms of Pl. cornutum in the genus Cyclonema. 



D'Orbigny made use of Stomatia for one of the Lower Silurian Platycerata. 

 Prodi-. I p. 29. In how far the genus Poly(o)stomella of R. Etheridge jr') can be 

 considered as related to Platyceras, I must leave undecided as I have not seen any 

 speeiinens belonging to it. On the other hand it is likely that some fossils have been 

 ranged with Platyceras, which in reality do not belong there, as for inst. some of 

 Sandberger's ^). 



The chief difficulties in the study of this genus are the dissimilar state of pre- 

 servation of the exteriör ornamentation of the shell and the almost incredible varia- 

 tions in its shape and growth. Some authors, as Morris in Qu. Journ. Geol. Soc. vol. 

 5 p. 332, write of a sinus in the outer lip of »Acroculia» nearly in the same sense as 

 the remarkable one in the Pleurotoniarida3. But besides that every sign of it is often 

 wanting in most specimens, there is by no means in those provided with it such a 

 great regularity as in the Pleurotomarida3, as there is only a wavy line, sometimes in 

 the Orthonychida3 several, corresponding with the indentures in the .apertural margin. 

 If there is more than one sinus there are also quite as inany wavy bands. These are, 



1) Proc. Roy. Phys. Soc. Edinburgh. 1880 vol. 5 p. 163. 

 -) Versteincr. Nassaus. pl. XXVI fig. 18, Cap. psiltacinus. 



