PACHYLASMA AURANTIACUM. 481 



Structure of Shell, fyc. — The inner surface (fig. 1 b) of the rather thick 

 parietes is smooth, except close to the base, where it is roughened 

 by a few irregular points. The sheath descends low down, and has its 

 lower edge slightly free. The sutures are much plainer on the internal 

 than on the external surface, and can be here plainly seen to be eight 

 in number. That portion of the carino-lateral compartment, which 

 helps to form the sheath, is narrow, though the whole compartment is 

 of very nearly equal width with the lateral compartment. It is a singular 

 fact, that no portion of the rostro-lateral compartment helps to form 

 the sheath; for the alae of the lateral compartments, overlap the whole 

 upper part of the rostro-lateral compartments, and abut against the 

 true rostrum. Hence, when the sheath alone is examined, the number 

 of the compartments appears only six. In a section the true rostrum 

 can be seen to underlie the rostro-lateral compartments, and thus 

 exhibits vestiges of alae. The fact of the rudimentary rostro-lateral 

 compartments not forming a part of the sheath offers a marked difference 

 from Chelonobia, which otherwise has this part of the shell very 

 similarly constructed. The diametric growth of the shell, which seems 

 to be considerable, is effected by the four large alee of the carina 

 and of the lateral compartment on each side. The sutural edges of the 

 alee are added to in a regularly inclined line down to the basis. The 

 basis is calcareous, and not very thin. 



Both opercular valves (fig. 1 c, Id) closely resemble those of the 

 last species. 



The Scuta are remarkably narrow and elongated : the external 

 surface is slightly furrowed longitudinally, the prominent lines of 

 growth are much wrinkled. The articular ridge is blunt, and not very 

 prominent. The upper part of the valve is reflexed, and a consider- 

 able portion must have projected freely. The Terga are nearly twice as 

 broad as the scuta : the carinal half of the valve is very smooth, and is 

 coloured beautiful reddish-orange. The spur is tolerably distinct. The 

 articular ridge is not very prominent. 



The Mouth resembles that of the last species. The mandibles have 

 three sharp teeth, with the inferior part narrow and pectinated with 

 long spines : the edge, also, between the upper main teeth, is pectinated 

 with short spines, which latter often have their summits crenated. The 

 maxillae are notched ; the inferior corner is produced into a small 

 step-formed projection. The outer maxillae are more pointed thanisusual. 



The Cirri most closely resemble in every detail those of P. giganteum; 

 1 can point out only one slight difference, namely, that the lower 

 segments, in the posterior ramus of the third cirrus, which are thickly 

 clothed with spines, are more numerous in proportion to the upper 

 segments with the spines arranged in regular pairs (being as 4 to 14), 

 than in P. giganteum, in which only one fifth (or 4 to 20) are so clothed. 



Caudal Appendages. — These are very small, not being more than 

 one fifth of the length of the pedicel of the sixth cirrus : their segments 

 are indistinct, and they support a very few coarse spines. The rami of 

 the sixth cirrus, in the one specimen, had twenty-five segments, whilst 

 each caudal appendage had only five. Hence the caudal appendages 

 are far less developed than in P. giganteum. 



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