332 
PALEONTOLOGY OF NEW YORK. 
similar in 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 spondylium 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 spoadylium 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 lamellae, and forms a receptacle for the proximal portion of the 
pedicler, 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 Orthes, 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 23edicle-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 
in 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- 
dyliuni appears to be but the complement of the deltidium, or the original 
plate formed uj^on 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 jhate 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 
