64 Memoirs of the Indian Museum. [Vol. ll, 



altogether absent but never showing any tendency to split up in 

 a secondary manner. When they are absent or degenerate the 

 membrane of the capitulum is greatly thickened ; a muscular layer is 

 sometimes present under the membrane. Lateral appendages normally 

 present, only absent in a very few instances; anal appendages absent or 

 consisting of a single claw-like joint, or multiarticulate. Mandibular 

 teeth usually pectinate. Prosoma well developed. Genera— Lß/)as, 

 Conchoderma, Heteralepas. 

 Subfamily (c). — Pgecilasmatin^. 



Species in which the five valves, unless they are degenerate, cover the 

 whole of the capitulum. If degenerate they tend to split up in a 

 secondary manner, the scutum being split into two by a vertical break 

 in most species. The valves may be almost entirely absent, but the 

 capitular membrane is not much thickened and is not lined by a 

 muscular layer. Lateral appendages absent ; anal appendages with 

 several joints. Mandibular teeth not regularly pectinate. Genera — 

 Pœcilasma, Dichelaspis , Megalasma. 

 Subfamily {d). — Alepadin^. 



Degenerate pelagic forms with transparent membrane devoid of a mus- 

 cular layer and with short, straight cirri. Valves absent or represented 

 by the scutum only. Filamentous lateral appendages absent ; anal 

 appendages absent or consisting of a single joint. Mouth parts much 

 simplified. Genera — Alepas (s. s.), Chœtolepas , Microlepas, Anelasma, 

 (?) Koleolepas. 

 All the genera included in this table are described in Gruvel's " Monographie " 

 except (i) Heteralepas, which has recently been established by Pilsbry (1907) for the 

 reception of most of the forms previously referred to Alepas and of several new 

 species, and (2) Microlepas, which has recently been discovered by Hoek (1907) in the 

 *'Siboga" collection. 



PART L— FAMILY LEPADID^ {sensu stricto). 

 INTRODUCTION. 



There are few groups in which the subdivision into genera and species is more 

 difficult than it is in the Pedunculata, and perhaps this is the case more particularly 

 as regards the Lepadidae than as regards the more primitive forms included in the 

 PolHcipedidae. I have already dwelt on the part played by convergence in the history 

 of the group as a whole, and it is perhaps in the Lepadidae that this phenomenon is 

 most manifest. Convergence, moreover, is here accompanied definitely by a marked 

 tendency to variation, which is strongest in those forms most degenerate as regards 

 their external characters (see plate vi). The early ancestor of the Cirripedes must 

 have been a free-swimming crustacean devoid of regular calcareous plates, which 

 developed on the integument of its descendants after they adopted a sedentary life. 



