DAVIDSOlir THIBET BEACHIOPODA. 37 



gists to consider some of its local variations in shape as distinct 

 species. The striae vary much in number and strength ; in some 

 British examples they are exceedingly numerous and fine, while in 

 other specimens they are much less numerous and coarser, the shell 

 differing in size much according to age and locality. G. variolata, 

 D'Orb., is, I believe, only a different name for the same shell. 



§ 2. JuEASsic Species. 



According to Capt. Godwin- Austen, rocks of the Jurassic period 

 are largely developed at Kato, in Ladak, in the Sum Country of 

 Thibet. In a Kmestone rock of a yellowish colour, which Capt. 

 Godwin- Austen refers to the Oxfordian period, the two following 

 species of Brachiopoda have been met with. 



1. Terebratijla Thibetensis, spec. nov. PL I. figs. 11-14. 



Shell longer than wide, sometimes oval and rounded in front, 

 but more often truncated or indented at the front, and even bipli- 

 cated. Yentral valve regularly convex and rather deeper than the 

 opposite one ; beak incurved and truncated by a circular foramen ; 

 dorsal valve evenly convex or biplicated near the front. Dimen- 

 sions variable. Length 1 inch 8 lines, width 1 inch 1 line, depth 

 11 lines. 



This species is extremely variable in its external shape, so much 

 so, indeed, that were I not provided with thirty-five or thirty-six 

 examples, showing every intermediate form, I should hardly ven- 

 ture to consider them all as variations in shape of a single species. 

 Some examples are of an elongated oval form (fig. 12), such speci- 

 m'ens being nearly uniformly convex with hardly any apparent bipH- 

 cation in the dorsal valve, under which aspect it resembles some 

 examples of T. punctata, and other closely related forms. Other 

 specimens have a straight or slightly indented hinge-line, vrith a 

 greater or less tendency to biplication, as in fig. 11, under which 

 condition they approach in shape some examples of Mr. E. E. Des- 

 longchamps's T. dorsoplicata and even T. perovalis, while others, 

 again, are decidedly biplicated, as in figs. 13 and 14, and are not 

 unlike some Cretaceous specimens of T. hijplicata and the Jurassic 

 T. indentata. 



The same kind of variation in form, with almost similar shapes, 

 has been shown to occur in T. biplicata and in T. dorsoplicata, &c. ; 

 and, indeed, so similar and perplexing are the resemblances of the 

 shell under description to several congeneric forms already described, 

 that it is with much uncertainty that 1 have ventured to apply to 

 the Thibet shell a distinct specific denomination. 



2. KHTKCHoisrELLA Katonensis, spcc. uov. PL I. figs. 15 & 16. 



Shell variable in shape, almost circular and globular, nearly as 

 wide as long. Yalves about equally convex and deep, ornamented 

 with from thirteen to fourteen angular ribs, of which from three to 

 six occupy a slightly elevated fold in the dorsal valve, and sinus in 



