64 PERMIAN FOSSILS. 



C. acuta differs from C. gihherula M'Coy,^ in being less convex, and having sharpei' 

 extremities. 



A single specimen from Byers' Quarry. 



No. 10. Ctthere (Cytherella ?) nuciformis, nobis. Tab. XVIII, fig. 11 a, h^ c. 



INCH. 



Length, ^ 

 Height, -xitj 

 Thickness, -^^^ 



Carapace oblong oval, smooth, convex. Dorsal and ventral borders elliptical. 

 Extremities rounded ; anterior depressed, rather narrower than the posterior. 



Dorsal aspect compressed ovate ; anterior oval. 



This little species has a somewhat similar form to that of Cytherella truncata. 

 Bosquet, of the Chalk.^ 



One specimen from Byers' Quarry. 



Order Phyllopa, Latreille, Qf 



Genus Dithyrocaris^ Scouler.^ 

 Argus, Scouler, 1835.® 



Gen. char. — Thorax protected by a carapace composed of two equal pieces that 

 are connected more or less closely at the dorsal margins. Tail (abdomen) of animal 

 protruding beyond the shell, having three caudal appendages. 



' Syn. Char., &c., p. 168, pi. xxiii, fig. 25. 



2 Jones, Mon. Entom. Cret. Form,, p. 30, tab. vii, fig. 25. Forms of carapace not very dissimilar to 

 C. inornata and C. nuciformis, occur amongst the Cytheres from the Carboniferous Limestone of Kilbride, 

 near Glasgow ; a suite of ■which, with other microscopic fossils, is preserved in the Museum of the College 

 of Surgeons of England. Seep. 58. 



3 The relation that the fossil genus under notice bears to the other Entomostraca is as yet uncertain ; 

 it not being determined whether the carapace is truly bivalve, hinged, and closing on itself, or spread open, 

 and liable to split in the mesial line. The Dithyrocarides were at first regarded by the discoverer as univalve 

 Entomostraca, allied to Apus, but subsequently Dr. Scouler has been induced to look upon the carapace as 

 bivalved, like that of Cypris, especially by the fact, as Dr. Scouler remarks, of the valves being often 

 found separated, in which case it is probable that the mesial line is a hinge. The opinion, however, of these 

 fossils having been univalved, i. e. of the valves having formed one unjointed whole, and presented a shield- 

 like carapace, as in Apus and Limulus, is adopted by Col. Portlock (Geol. Rep. Londonderry, p. 314), — an 

 opinion well supported by the appearance of D. Colei, and D. orbicularis, Portlock, and D. Scouleri, M'Coy. 

 Indeed it is still a matter of doubt whether the fossil remains in question do not belong to more than one 

 genus of Entomostraca. 



"* Etym. Ais, bis ; ^vpjj, valva; Kapis, squilla. 



^ See Portlock's Geol. Report Londonderry, p. 313. 



^ This name, having been preoccupied, was changed by Dr. Scouler to Dithyrocaris. 



