DESCRIPTION OF PLATES, 19, 20. 653 



by the fusion of the rostro-lateral and lateral compart- 

 ments ; e, carina ; b stands opposite one of the inward 

 folds of the wall, leading into one of the tubular apertures, 

 which runs (generally with an irregular spiral curve) 

 through the whole thickness of the shell down to the sur- 

 face of attachment. The carina has two of these apertures. 

 Fig. 4 6, Chamsesipho columna, another specimen, seen from the under 

 side, letters of reference the same. 

 4 c, Chamsesipho columna, rostrum, disarticulated, enlarged, show- 

 ing the alae. 



4 d, Chamaesipho columna, scutum and tergum, internal views of. 



5 a, Pachylasma giganteum ; 5 b, rostrum of an extremely young 



specimen, much enlarged, showing its compounded nature, 

 viz., of a true rostrum and rostro-lateral compartments ; 

 5 c, scutum and tergum, external views of ; 5 d, scutum 

 and tergum, internal views of. 



PLATE 20. 



Genera — Pachylasma, Octomeris, and Catophragmus. 



Fig. 1 a, Pachylasma aurantiacum, shell, lateral view of; A, rostrum, 

 separated only by the finest suture from B, the rostro- 

 lateral compartment, which latter has a mere rim-like 

 radius on the side facing the lateral (c) compartment. 

 The latter (c) has a great ala, and is separated from the 

 (d) carino-lateral compartment, by the finest suture ; d 

 has a rim-like radius ; e, carina, with very large alse (a). 

 1 b, Pachylasma aurantiacum, inside view of about half the shell, 

 showing the compound rostrum, consisting of the true 

 rostrum (a) and the two rostro-lateral (b, b) compartments; 

 of the latter, that to the left has a shoulder, receiving the 

 ala of the lateral compartment which has been removed ; 

 the ala of the lateral compartment extends, in the upper 

 part, over the whole width of the rostro-lateral compart- 

 ment, as may be seen on the right-hand, b ; c, lateral 

 compartment ; d, carino-lateral compartment, with a 

 shoulder (b) to receive the ala of the carina, which has 

 been removed. 



1 c, 1 d, Pachylasma aurantiacum, scutum and tergum, external 



vieM's of. 



2 a, Octomeris angulosa, shell seen from above ; 2 b, scutum 



and tergum, internal views of. 



3 a, Octomeris brunnea, shell seen from above ; 3 b, scutum 



and tergum, external views of. 



4 a, Catophragmus polymerus, shell seen from above. 



4 b, „ ,, external view of one of the 



scales or valves, from the 

 second whorl, counting from 

 the inside. 



