910 Report oF THE STATE GEOLOGIST. 
pods there are internal structures which seem to have served 
similar functions to the spondylium and cruralium of the 
Articulates. These occur in the genera LrneuLasma, TRIMBRELLA, 
Drxozotvs, and all the genera constituting the group termed by 
Davinson and Kine, the TrorereLtipa; and have been known 
as the platforms. Though all these structures have manifestly 
subserved the same purpose to the animal their origin is due to 
unlike causes though their growth was aided by similar con- 
ditions; hence they are not strictly homologous parts. 
The spondyliwm is an area of muscular implantation. It is 
derived from the convergence and coalescence of the dental 
lamellz and forms a receptacle primarily for the proximal por- 
tion of the pedicle and for the capsular or pedicle-muscles and 
eventually for all the muscles attached to the valve. For the 
inception of the platform on the pedicle-valve of the Trimer- 
ellids it is not so easy to assign an explanation, but its beginning 
once made, there seems no reason to doubt that the increase in 
size and prominence both of platform and spondylium has been 
greatly aided by the crowding of the essential organs of the 
animal about and beneath these muscular plates (see pp. 193, 
194). 
Considering the spondylium in its elementary condition where, 
as in Orruis, it is represented only by the convergent dental 
plates uniting with, or resting upon the bottom of the valve, 
enclosing only the base of the pedicle and its muscles, it becomes 
evident that the plate is actually but a modification of the 
original pedicle-sheath. It is the inner moiety of this .sheath 
surrounding the pedicle, which has become involved in or 
enclosed by the growthof the pedicle-valve, and further modified 
by the development of articulating processes where it comes in 
contact with the brachial valve. It may, therefore, be inferred 
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 has, at some period of growth, been accompanied by the exter- 
nal portion of the sheath which is termed the deltidium. Thus 
the spondylium appears to be but the complement of the del. 
tidium, and that the two were together included in the original 
162 
