(jo the VOYAGE OF H.M.S. CHALLENGER. 



not covered laterally by the five edges of the- rostral latera, seventeen have the rostrum 



laterally covered. 



For the right understanding of the nomenclature used for the valves and their margins 

 in the genus Scalpellum the woodcuts figs. 1 and 2 will prove useful. 



Fig. 1 represents a Scalpellum with a rostrum, and therefore with fourteen valves. 

 Fig. 2 one fill, out a rostrum, having therefore only thirteen valves. (A species with a sub- 

 carina is figured on PI. III. fig. 19.) In most valves the umbo is situated at the same 

 place in both figures. The exceptions to this rule are the upper-latus, infra-median latus, 

 and carina! latus. The upper latus, as a rule, has the umbo at the apex, as in fig. 1. In 

 Seal pell urn distiiictum, n. sp. and Scalpellum planum, n. sp. however, it is nearly in the 

 middle of the scutal margin. The infra-median latus, ordinarily, is triangular, and 

 in that case it has the umbo at the apex. Sometimes, however, its shape is that of an 

 hour-glass or of an elongate wine-glass on its stand, and then the umbo is seated near or 

 under the middle of the valve. Very interesting are the differences which the carinal 

 latus shows iu the different species. There are two types which are represented in the 

 two figures 1 and 2. In fig. 1 the valves of the lower whorl (the rostrum — when there 

 is one — the rostral latus, the infra-median latus, and the carinal latus) are not very much 

 developed in the direction of the long axis of the capitulum. On the contrary, the same 

 valves are much higher or more elongate in the other type (fig. 2). This influences the 

 shape of these valves, but specially that of the carinal latus. In the one (fig. 1 : type, 

 Scalpellum maximum, Darwin) the umbo is placed at the apex, as closely as possible to 

 the upper latus. In this case the whole of the carinal margin of this valve extends 

 beneath the umbo. In the other case (fig. 2 : type, Scalpellum vulgare, Leach) the 

 umbo is placed at a somewhat considerable distance from the apex, and then either at 

 the base of the carinal margin or about the middle of that margin. 



These two forms of the carinal latus were known to Darwin. It is curious enough 

 that all the recent forms known to Darwin have the carinal latus of the shape of fig. 2, 

 and all the fossil forms in which he was able to describe this valve — with the excep- 

 tion of one — show the type of fig. 1. However, this latter type is also represented in 

 the living forms ; a considerable number of the species inhabiting the deep-sea and 

 dredged by the Challenger give proof of it. There are in all thirteen species corre- 

 sponding with the fossil Scalpellum maximum with regard to the form of the carinal 

 latus, and nine of these inhabit a depth greater than 500 fathoms. On the other hand, 

 we must not lose sight of the fact that of the remaining twenty-nine species only three 

 were taken at a depth less than 500 fathoms; that the other type, therefore, is repre- 

 sented by twenty-six species in the deep-sea ! In the case of the present genus we find, 

 therefore, that the abyssal fauna consists partly of species resembling fossil forms, and for 

 a much more considerable part of species of a true shallow-water type. 



Nor does the study of the form of the carina give a more decided result. Darwin 



