Paheozoic Bivalved Entomostraca. 185 



to appear as minute pits, like the bases of hollow spines* 

 (fig. 4 a). In the Nassau specimen, referred to above, these 

 little pits are, in many of the hollow casts, very numerous, 

 strong, and regular. 



In thickness the riblets vary considerably ; so that we 

 may count 36 on some valves (figs. 2, 6) — and on others, 

 of about the same size, 30 (fig. 4), 28 (fig. 9), 22 (fig. 8), 20 

 (fig. 18), 18 (fig. 11), 16 (fig. 12). On smaller valves, with 

 the thin riblets, we find 22 (fig. 5), 18 (fig. 14), 16 (fig. 13) ; 

 and on a small valve of the thick- wrinkled kind there are 18 

 (fig. 10). 



As to the arrangement of the riblets, the typical pattern is 

 a parallel longitudinal folding, with turns of the middle riblets 

 (either backwards and forwards, or irregularly concentric) 

 near the ends of the valves, and a continuous concentric wrin- 

 kling at the margins. There are usually two terminal centres 

 of curves or folds ; but they may be modified or obliterated 

 by pressure. The whole series of riblets in some cases appears 

 to be naturally subcircular and concentric (figs. 2, 6, 9). 



The differences in essential characters among these Devo- 

 nian Entomides cannot be regarded as very great. The modi- 

 fications of shape can be recognized as distinctive only in so 

 far as some specimens (such as figs. 1, 2, 5, 7,13,14) are 

 more elongate and almond-like than others. Pressure in 

 several directions has interfered with the outlines, often to a 

 puzzling extent. The sculpturing has many gradations of 

 pattern. Perhaps the only tangible distinctions are : — (1) be- 

 tween the valves with strong (figs. 4, 8, 10, 11, 12, 18), and 

 those with feebler riblets — the former kind usually accom- 

 panying the oblong, and the latter the almond-like shape of 

 valve ; (2) among the latter, between the parallel (figs. 1, 5, 

 7, 13, 14) and the concentric arrangement of the costulation ; 

 (3) among the last, between the almost simple concentric 

 (figs. 2, 6) and the more labyrinthic (fig. 9) pattern. 



I have not been able to make out the intimate structure of 

 the valves as described and figured by Dr. Richter in the 

 * Zeitschr.' 1869 ; nor can 1 agree with the determination there 

 given of their parts and features. The " back- views " of the 

 test are to me the side-views of individual valves, not showing 

 the nuchal furrow, either because it is wanting on the out- 

 side of the valve (?), or because it is obsolete or obliterated by 

 pressure. The suborbicular forms, shortened by pressure 

 from end to end, were referred to in the memoir as " female" 

 individuals, and the nuchal or dorsal furrow as a transverse 



* Such a spinous surface, indeed, is indicated in Richter's ' lkitr. Pal. 

 Thiir. W.' pi. ii. f. U ; C. gyrata, p. 36. 



