310 Davidson — Earliest British Brackiopoda, 



I trust. I have been able to define its internal characters in a suffi- 

 ciently satisfactory manner. I was, moreover, able to demonstrate 

 that the so-termed Discina lahiosa had been established on the internal 

 cast of the ventral valve of the species under description. This I 

 ascertained, bej^ond doubt, first from finding the casts of both valves 

 of 0. Sagittalis abundantly spread over the same slabs; secondly, 

 because the casts agreed exactly in their resi^ective dimensions ; and, 

 thirdly, because, having, b}'- the aid of gutta percha, taken moulds from 

 these casts attributed to B. lahiosa, it became evident that the hollow, 

 supposed to be due to a foraminal aperture, was nothing more than a 

 prominence in the interior of the shell, as we have already described as 

 existing between the four muscular scars. Now, if we compare the ; 

 interior of the dorsal valve with the corresponding one in Crania, we 

 shall find in both the same large scars («), which in that genus 

 have been attributed to the divaricator muscle (Hancock), whilst 

 those marked (J) would have been produced by the occlusor or ad- 

 ductor muscle, and if the animal possessed ' anterior occlusors,' they 

 would, as in Lingula, occu2')y the sides of the projecting tongue- 

 shaped ridge at the place marked (c). Here, therefore, as in Crania, 

 the divaricator scars are larger than are the occlusor or adductor 

 ones. In Oholiis the scars (b) are larger than those marked (a), and 

 in addition to these on either of the lateral portions of the interior of 

 the valve are two other scars, not visible here. There is also a total 

 absence of hinge-area or groove for the passage of a peduncle, so 

 constant in Oholus. Nor do we find any trace of that flattened 

 internal margin which surrounds the valves in Crania. If we again 

 compare the interior of the ventral valve with that of Crania, we 

 should in both cases refer the scars (a) to the divaricator, while the 

 feebly-marked ones (b) would be attributable to the occlusor. 



Position and locality. — From the researches of Messrs. Hicks, 

 Salter, and Belt, this remarkable and characteristic s^^ecies appears to 

 be moderately plentiful throughout nearly the whole of the ' Menevian 

 group,' but it is still uncertain whether a minute, obscurely-marked 

 specimen found by Mr. Hicks in the upper portion of the ' Harlech 

 group,' and which underlies the ' Menevian,' may not belong to the 

 species under description. A still more minute shell, found tolerably 

 abundantly by Mr. Belt in the Lower Tremadoc beds of Craig-y- 

 dinas in North Wales, if not totally distinct from the present sj)ecies, 

 would at any rate constitute a well-marked variety, or even, perhaps, 

 species ; which we will, at least, provisionally, retain under the dis- 

 tinctive denomination of 0. Belti. 0. Sagittalis was obtained by 

 Mr. Hicks for the first time at Porth-y-rhaw, and subsequently at 

 Penpleidau, and several other places near St. David's, also by Mr. 

 Homfray in the beds of the Menevian formation at the Rheider 

 "Waterfall Valley and other places in North Wales. Mr. Belt obtained 

 it also at Gwynfynydd and in several other localities in the neigh- 

 bourhood of Dolgelly. 



Obolella Belti, Dav. PL XV. Figs. 25-27. 



Shell small, less than a line in length by about one line in breadth ; 

 transversely oval, beak acuminated, front broadly rounded. Valves 



