332 PALEONTOLOGY OF NEW YORK. 



similar iu aspect, are similar neither in origin nor function, and it becomes 

 necessary to modify the application of this term. Hence it is proposed to 

 restrict the term spondtjlium to the plate existing in the pedicle- valve, and to 

 the plates of the brachial valve, whether united or discrete, the name cruralium 

 will be applied. The distinction of the parts is necessary to a proper appre- 

 hension of their value. 



The spoadijlium is an area of muscular implantation. In its early or incipient 

 condition it is evident that it originates from the convergence and coalescence 

 of the dental lamellce, and forms a receptacle for the proxiiiial portion of the 

 pedicle, and for the capsular or pedicle muscles. In Clitambonites and Pen- 

 TAMERUS, where it attains its greatest development, it bears all the muscles of 

 the valve, the central adductor, and the lateral diductor scars being often clearly 

 defined, while the posterior portion of the plate is still reserved for the attach- 

 ment of the pedicle, if functional. Considering this structure in its incipient 

 condition, where, as in Orthis, it is represented only by the convergent dental 

 plates which usually unite with, or rest upon the bottom of the valve, and 

 enclose only the base of the pedicle and its muscles, it will be evident 

 that the plate is actually but a modification of the original pedicle-sheath. It 

 is, evidently, the inner moiety of this sheath surrounding the pedicle, which 

 has become involved or enclosed by the growth of the pedicle-valve, and fur- 

 ther modified by the development of articulating processes where it comes in 

 contact with the brachial valve. It therefore follows, as a natural inference, 

 that wherever the spondylium is present, whether in the incipient condition or 

 iu the more advanced stage of development in which it supports all the muscles 

 of the valve, it is, or, at some period of growth, has been accompanied by the 

 external portion of the sheath, which is termed the deltidium. Thus the spon- 

 dylium appears to be but the complement of the deltidium, or the original 

 plate formed upon the body of the embryo, and that portion of the adult shell to 

 which the term deltidium has been applied, is the other part of the original or 

 primitive deltidial plate or pedicle-sheath. Here again our terminology seems at 

 fault and should be further adapted to the proper conception of these structures. 

 Should the term prodeltidium be employed for the primitive body plate or the 



