260 



Mr. T. H. Withers on 



Carina (In. 18980). 



Length 

 Breadth 



would convey a very inadequate idea as to their size ; probable 

 measurements are tlierefore given: — 



Actual, Probable, 



in mm. in mm. 



12-4 130 



4-l(atabout4 50 

 mm. from 

 base). 

 12-3 14-2 (as fhown 



by impre3- 

 5 2 . . sion). 



3-3 

 20 

 14'4 17 



Carina (In. 18981) Length 



Subcarina (In. 18983). 

 Tergum (In. 18984) . 



Breadth 



Length 



Breadth 



Length 



Breadth (circa) 60 



60 



Description. — Carina semicylindrical, moderately bowed 

 inwards, strongly convex transversely, imperceptibly keeled 

 in its upper half, the valve tapering rapidly towards the apex, 

 which is sharply pointed ; basal margin moderately convex. 

 Outer surface marked with regular, widely-spaced, proniinen*, 

 raised ridges or zones of growth, which show, especially in 

 one valve (In. lt>981), a tendency to become broken up into 

 bead-like prominences. Fine transverse and longitudinal 

 lines are to be seen between the main ridges, but the longi- 

 tudinal lines are not so well marked as in the terga. The 

 valve is marked, especially in its lower part, with irregular, 

 closely set, tine puncta?. 



Scutum. — On the specimen In. 18986, lying near a right 

 tergum, wa.s a badly crushed and shapeless valve, evidently 

 showing its inner suiface. At its base could be discerned 

 one or two rather long and straight lurrows, and these 

 suggested to me that the fossil represented another kind of 

 valve. It was possible to clear away most of the miimte 

 fractured particles of shell, and there was then exposed some 

 eight or nine prominent, straight, equidistant furrows. A 

 plaster-cast taken from this impression shows that the furrows 

 represent the widely-spaced ridges or zones of growth such us 

 are seen on the carina and tergum above, except that they aru 

 straighter and longer, and there is no doubt that we have 

 here an impression of the outer surface of a scutum of the 

 type seen in the species known as PolVuipes aalensis^ Polli- 

 cipes ooliticiis, and Archceolepas quenstedii. 



Tergum subtriungular, slightly convex transversely, com- 

 paratively long and narrow, with prominent, widely-spaced, 

 transverse ridges, which form an acute angle of which the 

 a|»8x is situated about one-third the distance from the carinal 

 nnirgin ; there is no definite apico-basal ridge or fold. Carinal 



