334 PALEONTOLOGY OF NEW YORK. 



dylium, while the deltidium has been resorbed and secondary deltidial plates 

 or deltjiria formed about the pedicle-passage. 



Ill tlie fundamental division of the Articulata two groups will be recognized, 

 one embracing those forms in which the prodeltidiura is represented by the 

 deltidium and spondylium, one or both; the other a group in which the pro- 

 deltidium has been fully modified, resorbed or replaced. The former group is 

 equivalent to Waagen's suborder, Aphaneropeomata (1883), with the addition 

 of Thecidea and its allies, and to Beecher'.s Protremata (1891), excepting the 

 genus Tropidoleptcs. 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- 

 ameeus, CoxciiiDiUM, 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, RensseljEria, 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 



