52 THE VOYAGE OF H.M.S. CHALLENGES. 



Tergwm triangular, the occludent margin is only a little longer than the scutal 

 margin. The occludent margin shows the same flattened border as the larger and 

 uppermost part of the scutum ; on the contrary (as seen from PL VII. fig. 8), the bent- 

 down margin on the occludent side is not represented. As shown by fig. 6 (PL II.) 

 such a bent-down margin is very strongly represented along the carina! margin of these 

 valves, where it forms the continuation of the much flattened dorsal margin of the carina. 

 Basal angles pointed. The surface of the terga shows the same striation as that of the 

 scuta. Its lines of growth run parallel to the scutal margin. 



Carina externally almost entirely resembling the tergum (or the continuation of the 

 two terga). It is broad, especially towards its basal end, and much arched, running up 

 just between the basal ends of the terga. As I have already said, the dorsal margin is 

 much flattened, being broadest towards the upper end and gradually diminishing towards 

 the lower end. This dorsal flattened part is slightly but distinctly furrowed in a longi- 

 tudinal direction, the furrow forming the continuation of the margin between the two 

 terga. The surface is striated as in the other valves. The base is abruptly truncated. 

 At the inferior side the carina is barely concave, the undermost part being separated 

 from the larger uppermost part by an incomplete partition of a semi-annular form. 

 The latter resembles much the internal structure of the carina of Poscilasma carinatum 

 (PL VII. figs. 6 and 8). 



Peduncle very short, totally covered by the capitulum. 



Mouth. — Labrum with a row of teeth beneath the crest, which is bullate, and a pair 

 of slender palps with hairs at the internal surface (PL II. fig. 7). Mandibles (PL II. 

 fig. 8) with four teeth, the first standing a little way apart from the others, the fourth 

 close to the inferior angle, which is very little developed, and terminates in two micro- 

 scopical points. Maxillae, (PL VII. fig. 9) with two larger and two smaller spines on the 

 upper angle ; notch beneath the upper spines very large, and furnished with two not very 

 long and not very thick spines. Inferior upraised part with a double row of rather long 

 and thin spines. The whole maxilla has a very striking triangular form. 



Cirri. — First pair (PL II. fig. 9) far removed from the second pair, and about half 

 their length. Anterior ramus thicker than the posterior, the latter a little longer. Both 

 rami have eight segments. Numerous spines along both sides of the different segments, 

 and especially at the exterior face of the longest and thinnest ramus. 



Second pair with the anterior ramus a little shorter but not thicker than the posterior, 

 the number of segments of the longest ramus being thirteen, of the shortest eleven. On 

 most segments the number of pairs of spines along the margin is five. Of these the first 

 pair is extremely small, the second a little stronger, the three others of normal length 

 and strength. The anterior ramus, moreover, shows transverse rows of about four spines 

 on the lines separating the segments. 



Sixth pair with equal rami and both of thirteen segments. The arrangement of the 



