1 54 BALANID.E. 



imperfect outer lamina of the parietes towards their bases, 

 are characters which fail in one species of Platylepas ; the 

 muscles running to the opercular valves being thinly spread 

 out, and partially without transverse striae, is also a cha- 

 racter which fails in Platylepas ; the simplicity of the cement- 

 ducts partially fails in Tubicinella; and lastly, the exist- 

 ence of small intermediate teeth on the mandibles, fails 

 in Xenobalanus : hence, I repeat, I have not thought it 

 prudent to admit the sub-family of the Coronulinae, though 

 in many respects a very natural group. 



The genera in the Balaninae and Chthamalinse are 

 founded chiefly on the number of the compartments (the 

 number being apparently due, as previously explained, to 

 the fusion or abortion of certain of the eight typical com- 

 partments) ; and secondarily, on the nature and even form of 

 the basis, and on the porosity of the walls. In Coronula 

 and its allies, the non-articulated opercular valves and deeply 

 folded walls come into play. As a justification for using 

 these characters in distinguishing the genera, and even to a 

 certain extent in separating the two sub-families, I must 

 call to mind that the shell, with the basis, is not merely 

 a dermal envelope, as amongst Molluscs, but actually 

 consists of the first three segments of the head. The parts 

 of the mouth and the cirri are of very little service in distin- 

 guishing the genera, — a singular fact, considering that 

 most of the genera amongst the Lepadidse could be distin- 

 guished by these, organs, — though trifling details in their 

 structure sometimes come in useful as specific charac- 

 ters. Balanus, with the sub-genus Acasta ; Pyrgoma, with 

 the sub-genus Creusia ; Tetraclita, and Elminius, are genera 

 of about equal value ; though perhaps the two latter are 

 rather more nearly related together than to the others. 

 Chelonobia is more distinct ; it shows some little affinity 

 to the Chthamalinae, and likewise to the four following 

 genera. Coronula, Platylepas, Tubicinella, and Xenobalanus, 

 are genera quite distinct from the foregoing, and from each 

 other ; yet, as we have just seen, palpably allied together. 

 Amongst the Chthamalinae, Pachylasma, Octomeris, and 

 Catophragmus, are more closely related to each other than 

 to the other two genera of the sub-family; yet Pachylasma, 



