46 Rupert Jones, on Bivalved Entomostraca. 



from the strata of Germany, France, Belgium, &c. ; and at 

 home M'Coy, Salter, Kirkby, Holl, G. S. Brady, and myself 

 are among those who have treated of such as have been met 

 with in the British Isles ; but a large number still remained 

 undescribed. 



Amongst the fossil specimens are several that cannot be 

 readily co-ordhiated with the groupings made out of the 

 existing forms, as may be expected both by naturalists who 

 are accustomed to look on the existing races as successional 

 representatives of older forms, and by those who may regard 

 successive faunse as creational replacements. 



Among such fossil forms are many from the older (" Palaeo- 

 zoic ") strata ; but even for these existing representatives 

 occasionally turn up, such as Brady's Heterodesmus, lately 

 brought from the Japanese seas, which has apparently a 

 close affinity with M' Coy's Entomoconchus of the Mountain- 

 limestone. Some, indeed, of the old forms are scarcely dis- 

 tinguishable, as far as the valves are concerned, from their 

 modern representatives ; for instance, Cypridina primcBva 

 (M'Coy, sp.) of the same old limestone, and its associates 

 CypreUa and Cypridella, present in the various valves of their 

 multiform species gradations among themselves, and an easy 

 passage into Cypridina itself. Others among the ancient 

 faunae possess two or more of the characteristics that are 

 now divided amongst the several members of a group ; thus 

 the carapace of the Leperditia of the Silurian period has 

 resemblances in outline to members of the Limnadiadae, 

 Cypridininee, and Cypridge ; in muscle-spot to the first two ; 

 in vascular markings to the first and to the Apodidae ; in the 

 place of the eyes to the second and fourth ; and in the eye- 

 tubercles to the third and fourth. Altogether Leperditia, and 

 its palaeozoic congeners Isochilina,Entomis, Primitia, Beyrichia, 

 and Kirkby a, seem to be more nearly within the alliance of the 

 Lhnnadiadce than of the others. Nevertheless, in these as 

 well as in other groups of Bivalved Entomostraca, we have 

 ahvays to be careful in assigning special value to differences 

 of outline, ornament, and structure, because it is not unusual, 

 among these little Crustacea, to find that similar shells may 

 belong to different genera, when we examine them alive ; 

 and on the other hand very closely allied species may have 

 dissimilar valves. 



As a general rule the fossil Entomostraca of freshwater, 

 brackish, and marine strata, respectively, correspond in 

 family and generic characters to S])ecies found in such waters 

 at the present day ; and therefore the geologist often finds his 

 supposition as to the origin of a set of strata confirmed by the 



