232 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 I of the St. John group. These species are congeneric, and it is 

 possible that Mr. Walcott'sJ Orthis Eurekensis, from the Prospect Mountain 

 group, Nevada, belongs to the same genus. It has already been observed that 

 Billings' species, 0. Mycale and O. Tritonia, from the Levis formation, are also 

 without evidence of a cardinal process ; but it is impossible to say whether 

 they agree with O. 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 lamellag 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 terra 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 



* Habit, Dawson's Acadian Geology, Second Edition, p. (j44, fig. 223. 1S6S. Walcott, Bulletin No. 

 10, U. S. Geological Survey, p. 17, pi. i, figs. 1, 1 a-d. 1SS4. 



t Illustrations of the Fauna of the St. John Group, No. 3. 18S5. Ti-ans. Royal Soc. Canada, Sec. IV, 

 p. 43, pi. V, figs. 20 a-c. 1886. 



I Palseontology of the Eureka District, p. 22, pi. xi, figs. 8, 8 a. 1884. 



