64 LEPADID^E. 



Affinities. — Considering the close affinity between the 

 several genera, there are, I conceive, no grounds for 

 dividing the Lepadidse into sub-families, as has been 

 proposed by some authors, who have trusted exclusively 

 to external characters. In establishing the eleven genera 

 in the Lepadidse, no one part or set of organs affords 

 sufficient diagnostic characters : the number of the valves 

 is the most obvious, and one of the most useful charac- 

 ters, but it fails when the valves are nearly rudimentary, 

 and when they are numerous : the direction of their 

 lines of growth is more important, and fails to be charac- 

 teristic only in Scalpellum : with the same exception, the 

 presence or abscence of calcified or horny scales on the 

 peduncle is a good generic character. For this same 

 end, the shape of the scuta and carina, but not of the 

 other valves, comes into play. In three genera, the pre- 

 sence of filamentary appendages on the animal's body is 

 generic ; in Pollicipes, however, they are found only on 

 three out of the six species. The number of teeth in the 

 mandibles, and the shape of the maxillae, often prove 

 serviceable for this end; as does more generally the 

 presence of caudal appendages, and Avhether they be 

 naked or spinose, uniarticulate or multiarticulate ; in 

 Pollicipes alone this part is variable, being uni- and multi- 

 articulate; and in one species of Scalpellum they are 

 absent, though present in all the others. The shape of 

 the body, the absence or presence of teeth on the labrum, 

 the inner edge of the outer maxillae being notched or 

 straight, the prominence of the olfactory orifices, the 

 arrangement of the spines on the cirri, and the number 

 and form of their segments, are only of specific value. 



Comparing the pedunculated and sessile Cirripedes, it is, 

 I think, impossible to assign them a higher rank than that 

 of Families. The chief difference between them consists, 

 in the Lepadidse, in the presence of three layers of striaeless 

 muscles, longitudinal, transverse and oblique, continuously 

 surrounding the peduncle, but not specially attached to 

 the scuta and terga ; and on the other hand, in the Bala- 



