62 G. LINDSTROM, ON THE SILUKIAN GASTKoroDA AND |-TEH{)PU1)A OF GOTLAND. 



sequence of what ikjw has been adduced I cannot but consider Strophostylus also as 

 identical with Platyceras. 



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

 within many other genera of which some are enumerated here below. 



ExiHjyroceras a subgenus proposed by Meek and Woktiien in 1868, Geol. Survey 

 of Illinois vol. Ill p. 509 for shells with a sinistral spire and obscure C(jlumella. 



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

 330. It differs from the straight Platycerata (Orthonychia) only in having the surface 

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



yatica Adanson. Both Pictet and D'Okbigny think that this genus occurs in 

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

 sis (Sow. Sil. System pi. 5 A; 12 and D'Orbigny Prodr. I p. 29) are true Platycerata. 

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

 to this grou]j. 



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

 Platyceras. 



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

 created for the straight Platycerata, forming an elongate cone, with sauill 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 shown any reliable characters, by which 

 they may be separated from Platyceras. Meek and Woktiien, 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 PI. cornutum in the genus Cyclonema. 



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

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

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

 specimens 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 

 Sandbeeger's ^). 



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

 servation of the exterior 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 Pleurotomaridaj. 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 Pleurotomaridas, 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 many wavy bands. These are, 



') Proc. Roy. Phys. Soc. Edinburgh. 1880 vol. b p. 163. 

 -) Veisteiaer. Nassaus. pi. XXVI fig. 18, Cap. psiltacinus. 



