KNOWN SPECIES OF MARINE OSTRACODA. 377 



ages ; and, so far as I have been able to ascertain, the following changes may generally 

 be observed in tracing upwards a series of specimens from the youngest to the fully 

 formed adult. 



In the young state the valves are very fragile, pellucid, smooth, and much elongated ; 

 the anterior margin is produced into a wide, projecting, flattened border, and the pos- 

 terior margin is likewise considerably produced at its inferior angle : the three tubercles 

 on the lateral aspect of the valves are very large and conspicuous. This state, which 

 I propose to call var. nuda, is represented in PI. LIX. figs. 9 & 10. In a further 

 advanced stage of growth the surface-markings (pittings or reticulations) become 

 apparent, the tubercles more fused together and less prominent, and there is a wide, 

 flattened, marginal border nearly or quite round the shell. At the same time the 

 general outline is more rounded, and the terminal teeth begin to appear. In this stage 

 it is referable to C. latimarginata, Speyer (PI. LX. fig. 13, a-c). In the mature state 

 the teeth are more numerous ; the flattened marginal belt is, in great measure, lost ; the 

 reticulations or pittings are well developed, and the tubercular eminences less con- 

 spicuous. This stage of growth coincides with C. lyrata, Reuss, and is shown in PI. LIX. 

 fig. 12, a-c. In extremely old specimens the surface-markings become almost obsolete, 

 being either worn away by attrition or perhaps obliterated by constant deposit of new 

 shell in the interstices of the sculpture. The difficulty of investigating these closely 

 related forms has been considerably increased by the mutilated condition of most of the 

 specimens. It is seldom that I have been able to meet with perfect shells, the valves 

 being almost always separated. On this account the figures of the dorsal and ventral 

 aspects of the different varieties are not so complete as I could have wished. My 

 observations of their development have been made, therefore, to a considerable extent 

 on an undescribed British species {C.mutabilis, Brady, MS.) which is nearly allied to the 

 present. Several stages of the growth of this form are figured in PI. LIX. fig. 14, a-h. 

 The varieties there shown (which, it can scarcely be doubted, are dependent chiefly npon 

 stages of growth, but partly also upon habitat) have their precise counterparts amongst 

 the Norwegian and Arctic specimens, also figured in PI. LIX. 



13. Cythere lactea, n. sp. (PI. LX. fig. 3, a~c). 



Carapace quadrangular, convex. Extremities nearly equal ; anterior rounded, pos- 

 terior flattened and produced along its lower half. Dorsal and ventral margins 

 nearly straight, slightly sinuate. The lateral aspect of the valves exhibits a very large 

 rounded tubercle, connected with a posterior transverse ridge by a longitudinal emi- 

 nence. A flattened belt runs round the anterior margin, and is crossed at intervals by 

 short ridges. Dorsal profile angular, oval, very broad. Surface of the shell closely 

 reticulated. 



Length ^ in. ('63 mm.). 



Hab. Australia (17 fathoms, in sand). One specimen. 



VOL. V. PART V. 3d 



