the PaloBOzoic Bivalved Entomostraca. 61 



on the figure is an exaggerated feature, and without import- 

 ance ; in other respects the specimen somewhat resembles our 

 new Cythere Jukesiana^ but it is much shorter in proportion 

 and more arched. It also approaches some of the Bairdice in 

 shape ; but its narrow (anterior) extremity has the curvature of 

 a Cythere^ and is markedly deficient in the peculiar up-turned 

 hatchet-like edge characteristic of Bairdia. 



G. Aldensis is smaller (-V inch long) and less convex than 

 the specimen of Primitia Maccoyn associated with it in the 

 same limestone. 



The name " Oytherojpsis " has been applied to this and 

 other Palaeozoic Entomostraca*. With regard to this term as 

 a generic appellation, we once thought it useful in classifying 

 those Palaeozoic Entomostraca that do not closely assimilate 

 either to Leperditia or Beyrichia^ but in outline and size re- 

 semble many of the Cytherce of existing seas, though differing 

 from them in having tubercles, relatively thick valves, or other 

 distinctive features (Ann. Nat. Hist. ser. 3. vol. i. p. 249). The 

 establishment, however, of the natural group of Primitice en- 

 ables us to bring together several of the '^ simple BeyricMce^'' 

 some of the dubious Leperditioid forms, and nearly all the so- 

 called Cytheropses. Indeed of the known species referred to 

 CytJieropsis there remain only C. rugosa (Jones, Ann. Nat. 

 Hist. ser. 3. vol. i. p. 249, pi. 10. fig. 5), which is probably a 

 Primitia^ figured upside down, and C. siliqua (Jones, op. cit. 

 fig. 6), which, perhaps, like some of the Kildare specimens, is 

 a Cy there or a Macrocypris. Excepting the relatively greater 

 thickness of the valves in some of them (and that is more ap- 

 parent than real), there is nothing to indicate that these old 

 Entomostraca, which ^' Cytheropsis " was intended to com- 

 prise, dififered from what now exist as GytJiercej Bairdice^ 

 MacTocy prides^ &c. The so-called Cytheropses of the Car- 

 boniferous formations have already been shown to belong to 

 Leperditia Ohenij &c. (see Ann. Nat. Hist. ser. 3. vol. xviii. 

 p. 35). 



EXPLANATION OF PLATE Vn. 



Fig. 1. Primitia Maccoyii (full-grown) : a, right valve j h, dorsal^ and 

 c, ventral aspect. 



Fig. 2. P. Maccoyii (intermediate stage of growth) : a, left valve j 6, ven- 

 tral aspect. 



Fig. 3. P. Maccoyii (young) : a, left valve ; h, ventral aspect ; c, end 

 view. 



* ^'Cytheropsis " has also been applied to a group of recent Cytheridce by 

 G. 0. Sars in 1865 j but G. S. Brady proposes Eucythere in its place for 

 these living forms. 



