Fresidenfs Address. 07 



to the section Omphalotrochus.^ Similarly some species 

 occur in great abundance at various localities, for instance 

 Mr Young states that great quantities of Euom2')halus car- 

 honarins are often found together.-f 



We have hitherto dealt with Univalves possessing an entire 

 outer lip to the mouth, but we have now to consider a very 

 interesting group composed of rieitrotomaria and Murchisonia, 

 in which the outer lip is sinuated, and the whorls provided 

 with a revolving band. We may also regard here the genus 

 PlatyscMsma of M'Coy. These shells have usually been 

 referred to the Haliotidce, but it seems desirable to rani^e them 

 in a separate and distinct family, the Pleurotomariidce, thus 

 following several well-known authors.^ 



As it at present stands, FUiirotomaria (Defrance) is a most 

 unwieldy genus, and amongst its species presents great 

 modification of structure within certain limits. It contains 

 both umbilicated and imperforate shells, and many degrees 

 of variety are noticeable in the position and strength of the 

 spiral band, length of the sinus, and height of the spire. 

 Generic division is very desirable, and will have to be effected 

 on a combination of these characters. Unfortunately we are 

 here presented with an obstacle, only too common in such 

 cases — the extreme variability and inconstancy of the latter 

 throughout the range of species contained in the genus. 



One good subdivision, not usually recognised, has, I believe, 

 been made by d'Orbigny. Under the name of Polytremaria,^ 

 he has separated those forms possessing a constricted and 

 X^artly obliterated band, represented by oval foramina round 

 the whorls, some of which, at least, always remain open. A 

 Carboniferous species of this section is met with in Belgium 

 (P. catenata, De Kon.), but so ftir as I know is not found in 

 Britain. 



Two of our Carboniferous shells, tlmffdiv? striatus (Sby.) 

 and H. ? cirriformis (Sby.), and usually referred to Plcuro- 



* And may be identical with 0. tahulatus (Phill., sp.). 

 t " On the Gasteropod Mollusca of the Carboniferous Limestone of the 

 West of Scotland" (Proc. Nat. Hist. Soc, Glasgow, 1863, i., p. 70). 

 % See King, Perm. Foss., England, p. 213. 

 § Prod, de Pal., i., p. 122. 



