190 PALEONTOLOGY OF NEW YORK. 



a more or less developed cardinal process or callosity, which, in the typical 

 group, is elongate and usually simple at its termination. The dental sockets 

 are deep and their inner walls support short divergent crura. The muscular 

 impression is more or less distinctly quadruple, being divided longitudinally 

 by a median ridge extending forward from the cardinal process, and trans- 

 versely by fainter ridges. This quadruple scar marks the place of attachment 

 of the adductor muscle. 



Ramified vascular markings are often retained over the pallial area, the prin- 

 cipal trunks diverging from the median region and nearly following the curve 

 of the anterior margin of the shell. Between these and the adductor impres- 

 sions the markings of the ovarian areas are sometimes visible. 



Shell-structure fibrous and impunctate in the typical group. 



Observations. The foregoing diagnosis is based upon a stricter inter- 

 pretation of the generic characters than usually adopted by authors. The 

 very great number of species which are currently referred to the genus, and 

 the constant reference of new species of varied character to the same generic 

 term imply a want of homogeneity in the group, while in the present state of 

 knowledge it seems possible to indicate certain differential characters which 

 will serve as a basis for subsidiary classification. Differences in external con- 

 tour of the species are often extreme. The mind unwillingly associates the 

 transversely elongated, Spirifer-like forms of Orthis biforata with the round, 

 lenticular shells of Orthis testudinaria, the cordate Orthis biloba, or the inflated 

 and resupinate Orthis Macfarlanii. In the muscular markings also there are 

 some extreme variations; e. g., Orthis calligramma, with the scars obscure and 

 confined to the umbonal region, Orthis mvsculosa, having them very deep and 

 of immense size, covering almost the Avhole of the inner surface of the pedicle- 

 valve. The cardinal process varies in its form, size and degree of lobation ; the 

 dental lamellae are more or less prominently developed, and of still greater 

 importance is the fact that in its minute anatomy the shell is sometimes 

 compactly prismatic and impunctate, sometimes of looser texture and the inner 

 laminae abundantly supplied with vertical tubules. 



