334 
PALEONTOLOGY OF NEW YORK. 
dylium, while the deltidiuiii has been resorbed and secondary deltidial plates 
or deltaria formed about the pedicle-passage. 
In the fundamental division of the Articulata two groups will be recognized, 
one embracing those forms in which the prodeltidium is represented by the 
deltidium and spondylium, one or both; the other a group in which the pro- 
deltidiuni has been fully modified, resorbed or replaced. The former group is 
equivalent to Waagen’s suborder, Aphaneropegmata (1883), with the addition 
of Thecidea and its allies, and to Beecher’s Protremata (1891), excepting the 
genus Tropidoleptus. So deep-seated does this difference in these groups of 
genera appear, that examples of such combinations of primary and secondary 
conditions as shown by Camarophoria, are of the rarest occurrence. 
The spoon-shaped process of the brachial valve, which has been termed the 
cruralium, is a feature of more fugitive value. It is formed by the convergence 
or union of the crural plates, and it may rest upon the inner surface of the 
valve, or like the spondylium, be supported by a median septum. More often 
the crural plates, when highly developed, stand erect upon the valve and do not 
unite, but their position is highly variable, and it has been shown that in Pent- 
AMERUS, CoNCiiiDiUM, and their allied forms, the union of these plates is not of 
first importance as a generic character. When the crural plates extend to the 
bottom of the valve as distinct septa, they simply enclose an extension of the 
median incision of the hinge-plate. It has become evident, from a study of the 
hinge-plate, that the so-called visceral foramen which perforates it, and which 
is often present in Athyris, Rensselaeria, Cryptonella, etc., is a remnant of 
this aperture, the remainder of the median opening having become filled by 
a testaceous secretion. There is every reason to believe that the visceral fora¬ 
men was actually traversed by the lower alimentary canal, and if this were 
true, then the deep and narrow median chamber bounded by the crural plates 
must also have enclosed the terminal portion of the intestine. Within it lie 
the elongate scars of the adductor muscles, and when the chamber is elevated 
by the completed formation of a cruralium, these scars are still within it, as in 
the case of the spondylium. It is therefore the morphic equivalent of the 
spondylium. Its supporting median septum, when present, is composed 
