360 MR. G. S. BRADY ON NEW OR IMPERFECTLY 



And if little dependence can be placed on surface-markings, neither can we anymore 

 rely upon the spinous armature which is often observed at the extremities or round the 

 margins of the valves. These also vary much with age, and in the same gathering may 

 often be found either entirely absent or very well developed. Even the hinge-articula- 

 tion, which has been taken as a principal generic distinction, is liable to vary in degree 

 of development, not only in the same genus, but in different examples of the same 

 species ; so that, even in mature individuals it is often difficult to say to what genus 

 the species ought to be referred. This may very sufficiently account for the various 

 positions to which some species are assigned by different authors. 



The lucid spots afford, in some cases, excellent generic and possibly even specific 

 characters. Good illustrations of this are afforded by the genus Cytherella, by Jonesia 

 simplex, Bairdia fusca, Cytheridea kirkbii, &c. But the features on which most reliance 

 is to be placed are the general form and proportions of the carapace in its lateral and 

 dorsal aspects and the character (not the quantity) of surface- ornament. The proba- 

 bility of a small and slightly marked specimen being the young of some species is 

 always to be kept in view. Examples of young forms, which I at one time supposed to 

 be distinct species, are figured at PI. LIX. figs. 9 & 10 {Cythere clathrata, var. nuda), 

 ib. fig. 14 «, 6 {Cytkere mutahilis, Brady), and PI. LVIII. fig. 12 {Cythere setosa). 



For two groups which possess well-marked distinctive characters I have here proposed 

 the generic names Jonesia and Normania. The only other new genus which I have 

 found it necessary to establish is Heterodesmus — a highly curious and interesting nata- 

 tory form, allied to the fossil Carboniferous species Entomoconchus scouleri, M'Coy. 

 For this, as well as for the species of Cypridina now described, I am indebted to Arthur 

 Adams, Esq., R.N., by whom they were taken in the towing-net in the Japanese and 

 Chinese seas. My thanks are due also to Professor T. Rupert Jones and Messrs. W. 

 K. Parker, E. C. Davison, and W. M. Wake, who have kindly furnished me with most 

 of the specimens from which the following descriptions have been drawn up. 



It is with considerable hesitation that I have felt myself compelled to adopt the 

 genus (or subgenus) Cythereis. The species — our British C jonesii, for instance — which 

 may be looked upon as typical of that group, are so well marked and so peculiar in 



rule that a pitted sculpture on the lateral aspect of the valves is converted into furrows, more or less distinct, on 

 the ventral surface. In some species this is very well marked {Cythere septentrionalis. Sec), and appears to be 

 produced by a coalescence of the pits. Occasionally the process of formation of longitudinal furrows may be 

 observed in an intermediate stage {Cytkere hodyii). And, seeing that in some species there exist small elevated 

 tubercles on the spaces between the pittings, it is quite conceivable that, by the filling up of the excavations, 

 the shell might come to exhibit only small tubercles or papillae on a smooth surface. But I have seen no 

 examples in which this alteration can be traced ; and, if it were so, a species which had assumed a constant 

 character of its own by this means must have attained it by a long course of natural modification, and would, 

 I conceive, be entitled to rank as a distinct species. I have, however, observed that some species, which in 

 young and early adult life present a smooth surface studded with papillfe, become in old age encrusted with a 

 calcareous coat which exhibits pits or depressions in place of the papillae. 



