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PALAEONTOLOGY OF NEW YORK. 
sockets are obscure and the crural plates small; the latter appear to unite and 
form a low elevation across the base of the delthyrium. Cardinal process 
absent or rudimentary in all the specimens examined. Muscular markings in 
both valves extremely obscure. 
Surface marked by distinct plications, with interstitial finer radii, which are 
crossed by delicate concentric striae; these are usually accompanied by a low 
sinus and fold on the brachial and pedicle-valves, respectively; interior very 
finely papillose. Shell-substance fibrous and apparently punctate. 
Type, Orthis Billingsi, Hartt. St. John group. 
Mr. G. F. Matthew has kindly furnished an abundance of specimens for the 
study of Orthis Billingsi, Hartt, # and Orthis Quacoensis, Matthew,f from his 
Division 1 of the St. John group. These species are congeneric, and it is 
possible that Mr. Walcott’s $ Orthis Eurekensis, from the Prospect Mountain 
group, Nevada, belongs to the same genus. It has already been observed that 
Billings’ species, 0. Mycale and 0. Tritonia, from the Levis formation, are also 
without evidence of a cardinal process; but it is impossible to say whether 
they agree with 0. Billingsi in other respects. 
The characters of the St. John species are eminently comprehensive; first, 
the form of the shell is one more frequently met with among the strophome- 
nids than among the orthids; the concave plate formed by the union of 
the dental lamellae is never found in Orthis proper, though occurring in Scen- 
idium. In Orthisina or Clitambonites this plate is always present, but always 
supported by a median septum and invariably accompanied by the convex del- 
tidium, which, so far as known, does not exist in Protorthis ; while in the 
group typified by Orthis Pepina, Hall (here designated by the term Billingsella), 
the convex deltidium of Clitambonites is present and the concave or dental 
plate absent. The apparent absence of a cardinal process in Protorthis may 
be due to the imperfections of the fragile shells studied. The specimens of the 
* Hartt, Dawson’s Acadian Geology, Second Edition, p. 644, iig. 223. 186S. Walcott, Bulletin No. 
10, U. S. Geological Survey, p. 17, pi. i, figs. 1, 1 a-d. 1S84. 
t Illustrations of the Fauna of the St. John Group, No. 3. 1S85. Trans. Royal Soc. Canada, Sec. IV, 
p. 43, pi. v, figs. 20 a-c. 1886. 
% Palaeontology of the Eureka District, p. 22, pi. xi, figs. 8, 8 a. 1884. 
