84 



CARBONIJ^EROUS ENTOMOSTRACA. 



1881 Entomis, Jones, Geolog. Magaz., dec. ii, vol. viii, p. 341. 



1883 — Jones, Ann. Mag. Nat. Hist., ser. 5, vol. xii, p. 245. 



1884 — J. M. Clarke, Neues Jahrb. f. Min., &c., Jahrg. 1884, i, p. 184. 



Entomis has a bivalved, convex carapace, sub-oblong, ovate-oblong, fabiform, 

 amygdaliform, or subreniform ; and is more or less constricted dorsally by a transverse 

 furrow across the hinge-line. This impression, beginning on each valve at or in front of 

 the middle, is continued across its surface to the hinge-line, usually with a slight 

 curvature. In front of the sulcus there is sometimes a rounded tubercle ; but this is 

 variable in position and shape ; sometimes it is a spine, sometimes it is wanting. In 

 Entomis tuberosa^ a radiate musclc'Spot, somewhat like that of Leperditia, in connection 

 with the tubercle, is shown on some internal casts in Silurian mudstone from the Pentland 

 Hills, Scotland (Messrs. Haswell and Brown's collections). 



The surface of the valves may be either smooth, or ornamented with raised lines or 

 delicate riblets, arranged in a definite pattern, — transverse, longitudinal, or concentric, 

 and sometimes associated with minute spines or prickles. 



The anterior margin is not indented with any sinus or notch ; and is therefore without 

 beak or hood. 



This beakless and transversely sulcate form is, as above stated, of uncertain alliance. 

 It approximates to some of the beaked Cypridinads {Cyprella and Cypridelld) in having 

 a nuchal furrow (which is, however, traceable in many other Palaeozoic bivalved Entomos- 

 traca) and sometimes a sub-median knob ; but the former is usually very strongly 

 marked in Entomis, and the latter is very variable. In some species the sculpturing of 

 the valves distantly resembles the annular ornament seen in Cyprella (compare figs. 13 

 and 21, PI. IV) ; but usually it is quite distinct. 



As far as the shape of the carapace is concerned, Entomis stands in the same relation 

 to Ci/pridella and Cy])rella of De Koninck, as Polycope of Sars to Cypridina of Milne- 

 Edwards, the anterior notch having disappeared in both Polycope and Entomis. The 

 animals, however, may have respectively differed very much ; for Polycope and Cypridina 

 belong to different families, and the deep nuchal furrow in Entomis, far more impressed 

 than in Cypridella and Cyprella, was probably in direct relation with the structure of its 

 internal organs, under modifications not present in other genera. 



The physiological meaning of the nuchal furrow in these Entomostraca is not under- 

 stood. Among recent forms it is faintly indicated in Philomedes and Halocypris ; it is 

 stronger in Pleopsis and Da^jhnella, belonging to quite another group of Entomostraca. 



