360 MR. G. S. BRADY ON NEW OR IMPERFECTLY 
And if little dependence can be placed on surface-markings, neither can we any more 
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 [at one time supposed to 
be distinct species, are figured at Pl. LIX. figs. 9 & 10 (Cythere clathrata, var. nuda), 
ib. fig. 14 a, b (Cythere mutabilis, Brady), and Pl. 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, &c.), 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 (Cythere 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 ownby 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. Ihave, however, observed that some species, which in 
young and early adult life present a smooth surface studded with papillw, become in old age encrusted with a 
calcareous coat which exhibits pits or depressions in place of the papille. 
