CARBONIFEROUS AGE. 37 



or undulation*. Hinge edentulous ; ligament apparently wholly external. Dorsal 

 margin inflected so a* 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. 1'allial line faintly marked; its sinus sometimes deep, 

 rounded or angular. 

 Animal unknown. 



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

 which the shells of this genus can be al\\a\- readily distinguished from some of the 

 Triassie and Jurassic forms usually referred to J////r// x, and included by 1'rof. 

 Agassi/ in the groups for which he proposed the names Plevronn/n and MI/OJHU. 

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

 Jurassic rocks, would l>e at once referred to Mytn-!t<x, !'/> nromya, or Myopei*, by 

 most Geologists. As observed by 1'iof. 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 J/'/"y>- 

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

 the\ 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 

 Afyacitc*, from which the. genus under consideration would be mainly distinguished 

 by its edentulous hinge. The Allorismas arc, however, also generally longer shells, 

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

 as some species of Mi/m-Iti'*. 



From the genus P/i/<i</<i>//<t, to which this group is related, it can always be 

 distinguished by the total absence of the radiating costs 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 Allorlsma 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 Triassie and Jurassic rocks. 



Allorisma subcuneata. 



(FnxB I, Pig. 10, a, 6.) 



Allnriina ruticunrala, MRBK & HATDKK, Proceed. Acad. Nat. Sci. Phila. Deo. 1858, 263. 

 Comp. Sanguinotita rlara, McCoT, Brit. Pal. Fogs. 1852, Fasc. Ill, 604, pi. 3 F, flg. 12. 



Shell large, clavato-cnneate, gibbons in the anterior and nmbonal regions ; narrowed and compressed posteriorly. 

 Beaks depressed, incurred, 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 subtruncat* 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 :nul Map. Nat llist. Vol. XIV, 1844, p. 315, that it "is represented by Sanguinolaria 

 lulcata of Phillips." 



