110 P.ECILASMA FISSA. 



tion is hardly visible, and does not allow of movement ; 

 the fissure thus formed runs almost in the line connecting 

 the umbo and apex, (where in most species a ridge 

 extends,) but a little on the carina! side of it. The 

 occludent segment is narrowly bow-shaped, pointed at 

 both ends, with the upper end projecting slightly beyond 

 the apex of the lateral segment, and with the occludent 

 margin regularly curved from end to end. The lateral 

 segment is large, of an oval shape, with a narrow strip 

 cut off on one side. Primordial valves very plain at the 

 umbones of the lateral segments, but none are visible on 

 the occludent segments ; and this makes me believe that 

 these two pieces are normally parts of a single valve; 

 having only one specimen of P. Jissa, I was not able to 

 make out quite certainly whether the two segments are 

 continuously united at their umbones by a non-calcified 

 portion of valve, as is certainly the case with Dichelaspis. 

 The basal margin of the lateral segment is narrow, 

 inflected, and blends with the carino-tergal margin; it 

 has an internal, prominent, basal rim, and towards the 

 occludent margin a large, prominent; internal tooth. This 

 internal basal rim is not parallel to the outer basal 

 margin, but rises to a point a little way up the occludent 

 margin, in the same manner as in P. eburnea, but in 

 a lesser degree; in this latter species the peduncle is 

 internally almost cut off by the large disc of its carina ; 

 here, on the other hand, it is internally almost cut off by 

 these rims and the two large teeth of the lateral segments 

 of the scuta. 



Terga sub-triangular, short, nearly half as broad as 

 long ; three or four times as wide as the carina, and rather 

 wider than the occludent segment of the scuta; occludent 

 margin single, arched; carinal margin slightly arched; 

 basal angle bluntly pointed. 



Carina very narrow, much arched, running up just 

 between the basal ends of the terga ; exterior ridge en- 

 veloped in membrane ; heel blunt, prominent ; internally, 

 not concave, even slightly convex, produced at the lower 



