44 THE VOYAGE OF H.M.S. CHALLENGER. 



made a sketch of the isolated valves of a specimen of what I had previously identified as 

 Cryptoplax oculatus in. the British Museum, which perfectly agrees with my figs. 10a-10m, 

 and I am also greatly indebted to him for figs. 12a-12m, which represent the detached 

 valves of his species {Cryptoplax burrowi), also in the British Museum, which enables me 

 to illustrate all the species of Cryptoplax known to British zoologists. 



On comparing the anterior valve of the four species, it is seen that the tegmentum is 

 relatively largest in Cryptoplax striatus, and smallest in Cryptoplax larvarformis. The 

 posterior border of the articulamentum is greatly thickened in this species and in 

 Cryptoplax oculatus, but not so in the other species. This practically holds good also for 

 the remaining valves. 



In the seven posterior valves of Cryptoplax striatus, the anterior apex of the jugum of 

 the tegmentum always projects beyond the jugal sinus. In Cryptoplax burrowi and 

 Cryptoplax oculatus, it is more or less even with it, but in the latter, at least, it slightly 

 projects in some valves ; it never does so in Cryptoplax larvarformis, but a narrower or 

 wider space is always to be found in front of the tegmentum. 



The relative amount of articulamentum to tegmentum, exposed on a lateral view of 

 the posterior valve, varies considerably ; it is least so in Cryptoplax striatus, and most 

 uncovered in Cryptoplax larvwformis. Cryptoplax burrowi is unique in presenting the 

 tegmentum as entirely surrounded by the articulamentum when viewed from above. 



The sculpture of the three first species is, on the whole, very similar ; but in Cryptoplax 

 larvarformis the jugum is not definitely marked, and the surface of the shell appears to be 

 smooth, with concentric lines of growth. 



All the species, except Cryptoplax oculatus, are now known to be furnished normally 

 with sutural pores, and it is most probable that they will yet be found to occur in that 

 species. 



In most of the points above mentioned, a distinct series can be traced, Cryptoplax 

 striatus being the least specialised ; Cryptoplax burrowi follows, while Cryptoplax 

 oculatus is intermediate between this species and the highly specialised Cryptoplax 

 larvaformis. 



The British Museum contains specimens of only the four species above mentioned, 

 and, so far as I am aware, no other species occur in other British collections. Dr A. T. 

 de Eochebrune of the Museum d'Histoire Naturelle in Paris, has jriven short Latin 

 diagnoses of the following species : — Cryptoplax montanoi, n. sp., from Borneo ; Crypto- 

 plax per oni, n. sp., from Australia; Cryptoplax torresianus, n. sp., from Torres Straits ; 

 Cryptoplax caleclonicus, n. sp., Cryptoplax heurteli, n. sp., and Cryptoplax unciniferus, 

 n. sp., from New Caledonia. 1 It is impossible to determine what relation these species 

 bear to each other, and to the previously described forms, until we have figures of the 

 whole animals and of the isolated valves, in addition to full and minute descriptions. In 



1 Bull. Soe. PhUom. Paris, s<5r. 7, t. vi. pp. 190, 193, 195-197, 1882. 



