340 BRITISH SILURIAN BRACHIOPODA. 



to a foramen, was nothing more tlian the prominence in the interior of the shell, 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 {a), which in that genus have been attributed to the divaricator muscle (Hancock), 

 whilst those marked h have been produced by the occlusor or adductor muscle ; 

 and if the animal possessed anterior occlusors, they Would, as in Lingula, occupy the 

 sides of the projecting tongue-shaped ridge at the place marked c. Here, therefore, 

 as in Crania, the divaricator scars are larger than the occlusor or adductor scars. In 

 Obolus 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 Obolus. 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 shall in both cases refer the scars a to the divaricator, while the 

 feebly marked ones ib) would be attributable to the occlusor. 



Position and Locality. From the researches of Messrs. Hicks, Salter, and Belt, this 

 remarkable and characteristic species appears to be moderately plentiful throughout nearly 

 the whole of the " Maenevian Group," but it is still uncertain whether a minute, obscurely 

 marked specimen found by Mr. Hicks in the upper portion of the Harlech Group, which 

 underlies the " Maenevian,'' 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 species, at any rate 

 constitutes a well-marked variety, or even species ; and this we will provisionally retain 

 under the distinctive denomination of 0. Belli. 0. sagitlalis was obtained by Mr. Hicks 

 for the first time at Porth-y-rhaw, and subsequently at Pen-y-pleidiau, and several other 

 places near St. David's, South Wales. Also by Mr. Homfray in beds of the 

 Maenevian formations 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, North Wales. A species, nearly approaching in its characters to 

 0. sagitlalis, was found by Mr. Baily at Bellewstown Hill, County Mcath, Ireland ; also at 

 Balbriggan, County Dublin ; but the specimens were two obscure for correct identification. 



Obolella Belti, Dav. PI. L, figs. 15 — 17. 



Obolella Belt;, Dav. Geol. Mag., vol. v, p. 310, pi. xt, figs. 25 — 27, 1868. 



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

 transversely oval ; beak acuminated ; front broadly rounded. Valves moderately 



