4 THE VOYAGE OF H.M.S. CHALLENGER 



The shell consists of eight valves, of which the anterior and posterior invariably differ 

 from the six intermediate valves ; the anterior portion of the last valve resembling the 

 central areas of the intermediate valves, while its posterior portion resembles the anterior 

 valve ; this valve is thus naturally divided into an anterior area and a posterior area. 

 The intermediate valves are similarly divisible into two areas, the central area and the 

 lateral area. The former extends along the whole length of the ridge of the valve down 

 to the lateral border. The surface markings of the median dorsal line or jugum usually 

 differ somewhat from those of the lateral aspects of the central area, and for these areas I 

 propose and have adopted the term pleura (from -n-Xevpov, to, in plural, the ribs, side). 

 Each central area thus consists of a median jugum and a pair of pleura. The coloration 

 of the area is occasionally different in the two regions. The apex or umbo is the posterior 

 termination of the jugum, and rising more or less close to it, the lateral areas of the 

 intermediate valve diverge obliquely forwards. The surface marking of the lateral areas 

 of these valves usually closely resembles that of the anterior valve, and the posterior area 

 of the posterior valve. 



In this report I have adhered to the principle of describing the direction of the various 

 kinds of ornamentation in terms which bear relation to the animal as a whole. Thus, 

 longitudinal and transverse apply to the length and breadth of the animal, and the length 

 of an intermediate valve is its antero-posterior axis. Radiating markings are naturally 

 those which diverge from the umbo to the periphery. 



The exposed or upper surface of the shell is the tegmentum, which differs markedly 

 in its appearance and structure from the under surface. Although intimately connected 

 together, it is convenient to regard the shell as composed of two concentric and quite 

 distinct portions, the upper or tegmentum forming a cap which leaves the edges of the 

 lower or articulamentum more or less exposed. The tegmentum is raised above the arti- 

 culamentum, and at its periphery it usually more or less projects over the articulamentum, 

 forming the eaves or subgrundae. There is great diversity in the relative development of 

 these two layers of the shell, the articulamentum being most rudimentary in the Leptoidea 

 and having its maximum development in the Chitonelloidea. 



The articulamentum is in the intermediate and posterior valves produced anteriorly 

 into a pair of broad thin plates, the sutural laminse, the bay between them being the 

 jugal sinus (sinus jugalis). The lateral border of the articulamentum of the intermediate 

 valves is indented by a distinct notch, the lateral slit which demarcates the anterior from 

 the lateral insertion plates or side laminse of insertion. These are absent only in the 

 Leptoidea and Chitonelloidea. Except in the Leptoidea, the articulamentum of the 

 anterior valve always more or less projects, and is slit or notched, forming a variable 

 number of teeth ; the same occurs for the posterior border of the last valve except in the 

 Leptoidea, Schizoidea, Placiphoroidea, and Chitonelloidea. 



There is, as a rule, a considerable range of variation in the relative development of the 



