316 Mr. W. King on a new genus of Palaeozoic Shells. 



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

 Neither of the valves furnished with teeth. 



General Summary. — The various species at present known of 

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

 teral (extremely so in Allorisma (Sanguinolaria) undata, Portl.) : 

 their umbones are large in A. (Pholadomya) Munsteri (D'Archiac 

 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. elongata, 

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

 nob.). 



Allorisma in one essential point differs from every other genus 

 of Pholadomyidce : 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 Allorisma elongata, vertical in A. sul- 

 cata*, and intermediate in A. constrict a. 



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 separates them from the umbonial ca- 

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

 the pallial line is rather indistinct : in Allorisma elongata the 

 inflexion of the siphonal muscular impression is deep, and runs 

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

 as in Mya arenaria. 



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

 in some species {A. elongata and A. elegans, nobis) run in lines 

 from the umbones, but in others {A. constrict a) they are irregu- 

 larly arranged, as in Anatina subrostrata. 



Supplementary Notes. 



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

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



Allorisma elegans is a new species from the magnesian limestone 

 of Durham. 



Allorisma constricta is an undescribed carboniferous species from 

 Northumberland. 



The species called Allorisma sulcata (Hiatella, Flem., Sanguino- 

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



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

 given in the Monograph. 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 the umbone, and about an inch in length. 



