31G ]Mr. AY. King on a neic genus of Palceozoic Shells. 



dorsal plane of the shell to an internal and vertical position. 

 Neither of the valves furnished with teeth. 



General Summari/. — The various species at present known of 

 this genus are elliptical, equivalved, and more or less inequila- 

 teral (extremely so in AUorisma {Sajigidnolaria) iindata, Portl.) : 

 their umboncs are large in A. [Pholadomya) Munsteri (D^\rchiac 

 and De Verneuil), but small in others ; often they are strongly- 

 wrinkled parallel with their free margins, as in the genus Posi- 

 donomya : some appear to be closed at both ends {A. elonc/ata, 

 Mort.), while others gape anteriorly and posteriorly {A. constricta, 

 nob.). 



Alloi'isma in one essential point differs from every other genus 

 of Pholadomijidce : in the latter the cartilage fulcra are constantly 

 horizontal with the dorsal plane of the shell, — consequently they 

 support an external cartilage, whereas in the former they are va- 

 riable ; being horizontal in AUorisma elongata, vertical in A. sid- 

 cata^, and intermediate in A. constricta. 



The situation of the adductor and other muscular impressions 

 relatively to each other is nearly the same as in Thracia pubes- 

 cens : the anterior muscular impressions are so strongly marked 

 in some species {A. sulcata and A. undata) as to give rise to a 

 well-defined ridge which se])arates them from the umbonial ca- 

 vity ; in most of the species that have passed under my notice 

 the palhal line is rather indistinct : in AUorisma elongata the 

 inflexion of the siphonal muscular impression is deep, and rims 

 parallel with the ventral and tlie dorsal line of the shell somewhat 

 as in Mj/a arenaria. 



The surface of the shell is marked with minute pimples, which 

 in some species [A. elongata and A. elegans, nobis) run in hues 

 fi'om the umbones, but in others {A. constricta) they are irregu- 

 larly arranged, as in Anatina subrostrata. 



Supplementarij N'otes. 



Schlotheim's name Myacites implies that the shells so called are 

 fossil Myas ; as this is not the case the name cannot stand. 



AUorisma elegans is a new species from the magnesian limestone 

 of Durham. 



AUorisma constricta is an undescribed carboniferous species from 

 Northumberland. 



The species called AUorisma sulcata (Hiatella, Flem., Sunguino- 

 laria, Ph.) is the one figured by Professor Phillips in his ' Geology 



* A figure of this species, repvosenting one of its cartilage fulcra, will be 

 given in the JMonograph. I may just observe, that in a specimen of this 

 shell three inches long, the fulcra are nearly a quarter of an inch in breadth 

 a little behind tlie umbone, and about an inch in length. 



