320 CARBONIFEROUS LAMELLIBRANCHIATA. 



Fleming, sp., there has always existed considerable confusion between them. 

 King was of opinion that the shells described by Phillips and Fleming were quite 

 distinct, and referred Phillips's shell to Edmondia sulcata and Fleming's to 

 Allorisma sulcata. But considerable doubt exists that such was the case, and, 

 from the descriptions of both by Phillips and Fleming, I think it highly probable 

 that each had specimens of both shells before him when writing the description. 

 These shells certainly have a certain broad resemblance and may be easily 

 mistaken, but careful examination shows that they hardly possess a single 

 character in common. King described Fleming's shell under the name Allorisma 

 sulcata, which perhaps belongs to the family Grammysidee, for it has the typical 

 hinge and constriction from the umbo to the lower margin characteristic of that 

 family. The shell of A. sulcata is more oblique, has the umbones more anterior, 

 and the anterior umbonal limb rises gradually from the anterior edge of the shell, 

 and not by a distinct well-marked fold. The position of the anterior adductor 

 scar is different from that of E. sulcata, and there is no large accessory scar ; 

 moreover the pallial line in A. sulcata is deeply sinuated, and there is no ossicle 

 attached to the back of the hinge-plate, and the shell possesses a long escutcheon 

 and fairly well-marked lunule. Externally the ribs are simple and not double in 

 the anterior part of the valve. But with all the important differences, the general 

 appearance of the two shells is so strikingly similar, that the question naturally 

 arises as to what could have been the reason for such an external resemblance in 

 two such differently constructed animals. Is it possible that this resemblance is 

 due to protective mimicry, and that this factor of natural selection was already 

 exerting its influence in Carboniferous times ? Both species occur together, and 

 I have an idea that in any given locality one or other species is rare and the other 

 common. It is so at Lowick and Redesdale, but I have not sufficient evidence 

 from other localities to make the statement absolute. 



The descriptions given by Phillips and Fleming are so meagre that it has been 

 largely a matter of conjecture and of external evidence to decide to which shell 

 these authors were referring. King seems to have gone into the matter very 

 carefully, and referred some specimens to Fleming for comparison. Phillips, 

 however, thought that his and Fleming's species were identical ; and it is quite 

 possible that Fleming founded his species on specimens of both shells, for his 

 description is as follows : " Hiatella sulcata. — Beak nearly terminal, both extremi- 

 ties rounded, concentrically sulcated, ridges large retrally, formed by the union 

 of two or more ribs ; closely and obsoletely striated longitudinally, striae consist- 

 ing of minute tubercles." 



The beaks of Edmondia sulcata' are not nearly terminal, but neither are the 

 concentric ridges of Allorisma sidcata "large retrally, formed by the union of two 

 or more ribs." Internal casts of E. sulcata have obsolete radiating striae, which 



