536 Rev. G. F. WJiidhorne — Devonian Fossils, Devonshire. 



to the umbo ; from three to five being on the fold and five or six on 

 each side. 



Size : A specimen that has hardly been distorted is 40 mm. in 

 length and width, and 20 mm. in depth. A flattened specimen is 

 nearly 50 mm. long. 



This species is the prevailing shell of the locality. Very large 

 numbers of specimens have been found, but always in the condition 

 of casts. 



From Silurian to Middle Devonian (Kayser). 



Orthis hysteeita, Gmelin. 



1853. Orthis Beaumonti, Schnur : Palseontogr., vol. iii, p. 215, pi. xxxvii, fig. 9. 

 1889. ,, vulvarkis, Barrels : Ann. Sci. Geol. Nord, vol. iii, p. 72. 



1889. ,, hysterita, Kayser: Abh. k.p. Geol. Landes., n.s., pt. i, p. 53, pi. v, 



figs. 1, 7-9. 



Some casts of the double valves, showing the internal arrange- 

 ments, appear to agree exactly with the shell by Schnur referred to 

 0. Beaumonti, De Vern., and by Gosselet and Barrois to 0. vulvarius, 

 Schlot. ; the latter remarking that it is distinguished from 0. striatula, 

 Schlot., by its long and stronger muscular impressions. Other 

 larger casts of single valves equally correspond to Kayser's figures 

 of the same shell, for which, following Quenstedt, he adopts a still 

 earlier name. 



The shape of these fossils is a transverse oval, the dorsal valve 

 is deeply convex, the ventral valve is concave laterally and has 

 a broadly arched sinus. The valves meet in a deep sweeping curve 

 in front. The muscular area reaches rather more than half-way 

 forwards in the smaller examples, and less than half-way in the 

 larger. Only marginal traces of the very fine ribs remain. 



Size : about 30 mm. long by 45 mm. wide. 



Throughout the Lower Devonian of Germany. 



Orthis, sp. (PI. XVIII, Figs. 10a, &.) 



Cast longer than broad, tumid. Dorsal valve very convex, 

 larger than the other. Ventral valve apparently nearly flat, with 

 a wide shallow sinus in front and reflexed sides, and massive near 

 the umbo with a wide oblique hinge-area. Hinge-line as long as 

 the width of the shell. Cardinal angles gently rounded. Valves 

 meeting in front in a sweeping curve. Muscular impressions 

 extremely large and strong, reaching very nearly to the front margin 

 of the shell in both valves. Surface covered with very numerous 

 small rounded striae, which seem to divaricate close to the margins. 

 Shell-structure thick. 



Size : about 20 mm. long, 15 mm. wide, and 11 mm. deep. 



There is a cast of the closed valves and an exterior of the dorsal 

 valve, which retains the surface though much decayed. These 

 specimens, together, show a good deal of the character of the 

 species, which seems to me very distinctive. Internally, it appears 

 very like 0. Monnieri, Eoualt, from the Lower Devonian, but it 

 differs from it in shape and in the size of the hinge-area. I have 

 seen very similar specimens from the Lower Devonian of Cornwall. 



