BRACmOPODA. 
333 
pedicle-sheath in its entirety, we shall then have the terms spondylium and delti- 
dium applied to corresponding and equivalent modified parts of this plate, the 
former internal, the latter external. 
The adult condition of the shell does not always furnish complete, and some¬ 
times not even suggestive evidence of the relations of the spondylium and 
deltidium. For example, in the genus Orthis and its various subdivisions, the 
delthyrium is almost always open at maturity and indeed all through the later 
growth-stages of the shell. The deltidium unquestionably e.xisted at an early 
stage and has usually become resorbed long before evidences of maturity in 
other respects are assumed; the spondylium, also, does not pass beyond a 
condition which makes the pedicle-cavity a clearly defined feature of the inte¬ 
rior. Tn more elementary or less modified orthoid structures like Billingsella, 
Protorthis, and the Orthis dejiecta and 0. loricula (see Plate V a, figs. 30, 31), 
the deltidium is fully retained at maturity, while the spondylium remains in its 
condition of a simple pedicle-cavity. The coexistence of both features with a 
high degree of development, as in Clitamronites, Polytcechia, etc, indicates a 
more primitive condition than in Orthis, though in such cases the extension of 
the spondylium to such a degree as to carry all the muscular bands of the 
pedicle-valve must be regarded as a secondary modification of this organ. In 
Pentamerus and allied genera, where the spondylium attains its greatest de¬ 
velopment, the deltidium is usually lost, but when retained is very thin and has 
a concave exterior, a form doubtless largely due to the arching of the umbo of 
the pedicle-valve over the full, procumbent beak of the brachial valve. The 
spondylium occurs in various modified conditions; in cases where the teeth are 
wholly without dental lamellae, or where such lamellae do not extend to the 
bottom of the valve, it seems necessary to regard them as instances of 
degeneracy or resorption of the primitive spondylium. As the growth, modifi¬ 
cation and disappearance of the differential parts of the prodeltldium do not 
progress pari passu, there will frequently be examples of one being retained 
when the other has disappeared. A remarkable illustration of this fact is 
afforded by the genus Camaropiioria, which pos.sesses a highly developed spon- 
