NEW CARBON] FEROrS MOLTAJSKS—CTRTY. 725 



Zf)/rlpeoteii oecide/italis re^resiints the thin external lay<'i- whose struc- 

 ture in the slides of Z. texanns had been obliterated. If these ol).serva- 

 tions are correctly correlated, the presence in these shells of an inner 

 kniellose, pro])ably nacreous layer, and of an outer tubulous layer, is 

 important in ascertainino- their relationship with other ocuei-a. 



Another form in which the hinge structure characteristic of Llmi- 

 pecten has been o])served seems to be as yet undescri))ed. It was found 

 .near Topeka, Kansas, and is allied to L. occidental ix, from which it is 

 distinguished chiefly by its large Hat ril)s. No new facts are added 

 by observations made upon this species. 



The thii-d and last form, supposed to show the hinge structure of 

 Z'mi/?(?c?;6;w, is a species identitied by De Koninck with Avlcidlpecten 

 ca'latiia McCoy. His figure represents a hinge plate with a liganiental 

 pit similar to that of Lhiiipectcn^ and his descri})tlon indicates a similar 

 median structure, although the interpretation is certainly ditt'erent. 



All these shells are more or less nearly erect; but because imperfec- 

 tions in outline are frequent, and sometimes ver^' misleading, it is 

 seldom possible to tell contidently wdiether the axis is directed forward 

 or backward. Both conditions seem to occur; and while in some cases 

 this character ma}- serve to discriminate species, I doubt if it can be 

 applied to generic groups. In practice its utility for any rank of 

 discrimination will of necessit}' be much limited. In my tigures 

 LhirijMicten texanus is represented as having a forward inclination, but 

 I am not altogether satisfied as to the fact. The growth lines seem to 

 indicate this shape, but the circumstance that the cartilage pit has a 

 strong backward inclination may not be without signiticance. 



Aside from the structure of the hinge plate, several features in the 

 configuration and ornamentation of LvmijX'cten are more oi" less strik- 

 ing. The inequivalve character of the shell is one of these. The right 

 valve throughout the forms referred to Limipecten is not only ver}' 

 much flatter than the left, but has somewhat different and much fainter 

 surface ornamentation. The wMugs are not always conspicuously 

 unequal. If anything, the posterior one is larger than the anterior; 

 this fact being determined by the distance from the point of the umbo 

 to the extremity of the wing. \\\ the typical species the anterior 

 wing is strongly defined, and the posterior undefined, but while it 

 seems to be a general truth that the demarcation of the anterior wing 

 is more a])rupt than that of the posterior, individual specimens can l»e 

 found, as, for instance, in L. occldentfdts, in which there is little if any 

 difference, both wings being strongly outlined. In L. fcnniKs the 

 plications, faint over most of the shell, are practically ol)solet<^ on tiie 

 wangs, though traces of them were noticed in individual specimens. 

 In Z. occidentalism a strongly radiate species, the ril)s are distinct u^^on 

 the wings, being stronger upon the anterior wing (in the right valve) 

 than upon the posterior. The relative size, demai-caf ion, and oi iiaiU(Mi- 



