214 CAMBRO-SILURIAN FOSSILS. 



few straight, once- or twice-branched impressions of the pallial 

 vessels on each side. Length 9 hnes, proportional length of 

 entering valve y^^^Q, width -J-^^y depth of both valves -f^-^. 



The general ovate form of this species, with its narrow front, 

 without mesial ridge or sinus, and its pointed beak, with large 

 foramen, easily distinguish it from any of the other Lower Palaeo- 

 zoic forms. It varies considerably in depth, being sometimes 

 rather less than the above dimensions, and having in one speci- 

 men the proportion of j^^, compared with the length ; the width 

 is sometimes a little more and sometimes a little less than the 

 measurement given above. Some specimens seem to show pro- 

 jecting irregular transverse squamae of growth. 



Occurs in great number in the greenish sandstone of Mulock 

 quarry, Dalquorhan, near Girvan, Ayrshire. 



[Col. University of Cambridge.) 



Hemithyris Davidsoni (M*Coy). 



Ref. & Syn. Terebratula sphcerica (David.), Bull, de la Soc. Geol. 

 de France, 2nd S. vol. v. t. 3. f. 36 (not of Sow. S. S. nor Sow. 

 Geol. Trans.). 



Sp. Char. Globose or subcuboidal ; entering valve deeper than 

 the receiving one, moderately convex on the rostral half, ab- 

 ruptly and obtusely bent over towards the margin, nearly at 

 right angles ; beak of the receiving valve small, with a minute 

 triangular opening beneath the apex, moderately convex, de- 

 pressed towards the margin to form a broad, slightly defined 

 sinus, the corresponding ridge to which in the entering valve 

 is scarcely elevated ; lateral margins nearly straight, front 

 margin elevated into a quadrate sinus twice as wide as high ; 

 valves radiated with from eighteen to twenty very obtusely 

 angular simple ridges, reaching distinctly to the beak, and 

 sharply notching the margin, the four middle ones perceptibly 

 larger than the rest. Length and width equal, and about 5 

 lines, depth of deflected front -^^q, depth of both valves j^^^^. 



This species differs from the Terebratula sphcerica (Sow.), Sil. 

 Syst., by having the elevation of the front towards the entering 

 valve as in ordinary cases, instead of towards the receiving one as 

 in that species, which in consequence of this Mr. Davidson sug- 

 gests to be a variety of H. deftexa ; and the Devonian Atrypa 

 sphaerica (Sow.), to which the present fossil is referred by Mr. 

 Davidson, differs completely by having all the rostral portion 

 smooth, while the ridges extend clearly to the beak in this Silu- 

 rian type. The general form slightly recalls that of the H. Wil- 

 soni, but there are much fewer ribs, and they are not divided by 

 a medial sulcus on the deflected front. 



