190 
PALAEONTOLOGY 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 musculosa, having them very deep and 
of immense size, covering almost the whole 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. 
