SCALPELLUM. 25 



point, from the lateral margin, which in the lower half is slightly protuberant ; tergal 

 margin straight, with the edge thickened and sHghtly reflexed. A distinct, square-edged 

 ridge (therefore formed by two angles) runs from the umbo to the baso-lateral angle, which 

 is itself obliquely truncated. Internally (/), there is a large and deep pit for the adductor 

 scutorura. Terga (fig. 4(5) triangular, flat, large, fully one third longer than the scuta; 

 basal half much produced ; basal angle pointed ; from it to the apex or umbo there runs 

 a narrow, almost straight furrow, at which the lines of growth converge — it runs at about 

 one third of the entire width of the tergum (in its broadest part) from the carinal margin. 

 Parallel to the occludent margin, and at a little distance from it, there runs a wide, very 

 shallow depression up to the apex. The scutal margin is not quite straight, about a third 

 part, above a slight bend corresponding with the apex of the upper latera, being slightly 

 hollowed : from the above bend a very faint ridge runs to the apex of the valve. Upper latera 

 (fig. 4 d) large, fiat, with five sides, of which the two upper are much the longest ; the 

 basal side is next in length, and the scutal side much the shortest. As far as I can judge 

 of the positions of the lower valves, with respect to the upper latus, I believe, that the 

 rostral latera, probably, abutted against the shortest of the three lower sides ; that the 

 carina ran along the one next in length, and the carinal latera along the middle basal 

 side, which I suppose extended in an oblique line, and not parallel to the base of the 

 capitulum : the two upper long sides no doubt touched the scuta and terga. The umbo 

 of growth is at the apex ; there is, however, a trace of a projecting ledge added round the 

 upper margins during the thickening of this upper part of the valve. Two slight ridges run 

 from the apex to the two corners of the middle of the three lower sides. Carinal latera 

 (fig. 4 (?) : these are not quite perfectly seen : the umbo forms a sharp point, whence the 

 valve rapidly expands and curves apparently downwards and towards the upper latera. 

 Near one margin there is a very narrow furrow, and on the other a wide depression, both 

 running and widening from the umbo to the opposite end, which is slightly sinuous. I 

 imagine these carinal latera occupied a nearly triangular space between the middle of the 

 three lower sides of the upper latera and the basal portion of the carina. Rostral latera, 

 rostrum and peduncle unknown ; the rostral latera must have been very narrow. 



Affinities. — In the shape and manner of growth of the scuta, and more especially of 

 the upper latera, this species is certainly more closely allied to the eocene S. quadratum, 

 than to any other species ; but in the peculiar characters of the carina, it is nearer to the 

 recent S. rutilum ; we have previously seen that the nearest congener to S. quadratum is 

 this same 8. rutilum. The most conspicuous diagnostic character of this species is derived 

 from the peculiar form of the carina, — its tectum being bounded by a rounded ridge on 

 each side. The square-edged ridge running from the apex to the baso-lateral angle of 

 the scuta is a trifiing, but I believe, a diagnostic character. If I am right in placing 

 8. rutilum in the genus Scalpellum, and I think there can be no doubt of this, considering 

 the characters of its complemental male, then there can be no question that the present 

 species belongs to the same genus. 



d 



