CRANIAD A. 79 
or adductor muscle. A narrow longitudinal ridge in the shell (depression on the cast) 
commences between the last-described impressions and extends to some distance in the 
direction of the anterior portion of the shell, and on either side of its extremity may be 
seen a small oval scar, which has been supposed attributable to the brachial muscle. The 
internal concave space beyond the visceral portion of the shell, and within the flattened 
margin or anterior half of the valve, is covered with narrow radiating ridges, bifurcating 
at their extremities, and with broader interspaces between them. ‘The interior of the 
ventral valve differs from the dorsal one by having its divaricator scars much narrower, 
and its occlusor impressions closer at the anterior extremity, and leaving only a very 
small space between them for the two little scars which Mr. Hancock thinks might belong 
to the anterior extremity of the dorsal adjustor. The occlusor or adductor muscular 
impressions in this valve are slightly concave, but projecting in the cast, and the remaining 
concave space is, as in the opposite valve, covered with slightly raised ridges, with wide, 
slightly concave interspaces. 
At page 25 of his ‘Synopsis of the Silurian Fossils of Ireland,’ published in 1846, 
Prof. M‘Coy refers, with uncertainty, the shell under description to the Crania anti- 
guissima of HKichwald; and subsequently, at p. 187 of his ‘ British Palaeozoic Fossils,’ he 
observes that ‘‘in size and general character this shell agrees with Pseudo-crania antiquissima 
(Hichw., sp.) as given by M. de Verneuil (‘ Geol. of Russia, vol. ii, pl. i, fig., 12), but is 
easily distinguishable externally by the beak being close to the posterior margin and by 
the remarkably divaricating sculpture of the valves, and internally by several minor 
points of detail obvious by comparing the figures.” I quite coincide with the distinguished 
Irish paleontologist, the Irish and Russian forms being specifically distinct ; the last is 
smooth externally, while the former is striated or costate ; but not feeling satisfied with 
M. de Verneuil’s figure of the interior of C. antigquissima, as it showed no divaricator 
impressions in the shell, I obtained from my friend Dr. Volborth, of St. Petersburg, two 
well-preserved examples of the Russian species, which showed in the interior of the ventral 
valve two distinctly marked narrow divaricator scars, very similar in shape and _ position 
to those seen in our own British species. The principal internal difference consists in the 
occlusor or adductor scars being very prominent in Eichwald’s species ; while they are, on 
the contrary, slightly concave in the British shell. Both species, however, may be placed 
in the same section of the genus Crania (unattached species). 
Prof. M‘Coy figures the exterior and internal casts of the ventral valve only, but in 
our plate will be found the interior of both valves, enlarged so as to show all their inte- 
rior characters. 
Mr. Baily’s figure of the internal cast of C. catenulata cannot be quite correct, for 
the divaricator impressions, besides being out of shape, are too large in proportion to the 
occlusor or adductor marks. 
Position and locality. Mr. Salter first collected this shell, and described it as 
C. catenulata, at the Meeting of the British Association at Cambridge, 1846. It is 
