CARBONIFEHOTJSAGE. 37 



or undvilations. Hinge edentulous ; ligament apparently wholly external. Dorsal 

 margin inflected so as to form a lanceolate depression or false area along the car- 

 dinal border behind the beaks. Scar of anterior adductor muscle occupying a 

 comparatively low position. PaUial line faintly marked ; its sinus sometimes deep, 

 rounded or angular. 

 Animal unkno'wn. 



We are rather at a loss to find well marked and constant external characters by 

 which the shells of this genus can be always readily distinguished from some of the 

 Triassic and Jurassic forms usually referred to Myacites, and included by Prof. 

 Agassiz in the groups for which he proposed the names Pleuromya and Myopsis. 

 Indeed some of our Devonian and Carboniferous species, if found in Triassic or 

 Jurassic rocks, would be at once referred to Myacites, Pleuromya, or Myopsis, by 

 most Geologists. As observed by Prof Agassiz, the shells included by him under 

 the latter two names are very closely allied, and it was mainly in consequence of the 

 presence of cardinal teeth, and a granulated surface in several of the species of Myop- 

 sis (characters not observed in those referred to the group he called Pleuromya), that 

 they were separated. Some subsequent European investigators, however, say they 

 find these characters common to species included in both groups. If these observers 

 are not mistaken, these two groups should probably be united under the older name 

 Myacites, from which the genus under consideration would be mainly distinguished 

 by its edentulous hinge. The AUorismas are, however, also generally longer shells, 

 with more depressed beaks, and they were probably never so widely gaping behind 

 as some species of Myacites. 



From the genus Pholadomya, to which this group is related, it can always be 

 distinguished by the total absence of the radiating cost« so characteristic of that 

 genus. They likewise differ in the granulated character of the surface, though it 

 is rather rarely the case that we find specimens in a condition to show this latter 

 peculiarity. 



The genus Allorisma appears to have been first introduced during the Devonian 

 epoch, and attained its maximum development before the close of the Carboniferous. 

 It also occurs in the Permian rocks, and, as already stated, some very similar forms 

 have been described under the name of Myacites from the Triassic and Jurassic rocks. 



Allorisma subcuneafa. 



(Plate I, Fig. 10, a, 6.) 



Allorisma subcuneata, Meek & Hayden, Proceed. Acad. Nat. Soi. Phila. Dec. 1858, 263. 

 Comp. Sanguinolites clava, McCoy, Brit. Pal. Foss. 1852, Faso. Ill, 504, pi. 3 F, fig. 12. 



Sliell large, clavato-ouneate, gibbous in the anterior and umbonal regions ; narrowed and compressed posteriorly. 

 Beaks depressed, incurved, and located about one-eighth the entire length of the shell from the anterior extremity. 

 Posterior end narrowly rounded, and apparently moderately gaping ; anterior end obliquely subtruncate above, and 

 rather narrowly rounded and somewhat produced below ; basal margin nearly straight along the middle, contracting 



This, however, was unnecessary, since he had distinctly stated in first publishing the genus in the 

 Annals and Mag. Nat. Hist. Vol. XIV, 1844, p. 315, that it "is represented by Sanguinolaria 

 sulcata of Phillips." 



