200 Mr. T. H. Witliers on some 



agree with those of Brachylepas in number and disposition, 

 and, except for the narrower carina and rostrum, in their 

 structure also. The narrowness of the carina and rostrum, 

 however, is of significance, for in this character they agree 

 more with the pedunculate Cirripedes, Brachylepas is 

 considered to he a sessile Cirripede, mainly because of the 

 modification of the basal whorls of imbricating plates to 

 form a shelf or platform round the base of the capitulum; 

 and the much wider semiconical carina and rostrum allow of 

 a closer approach of the capitulum to radial symmetry, 

 which is in accord with this interpretation. 



The great resemblance between the capitular valves of 

 Brachylepas and those of the series of species included in 

 Pycnolepas suggests the ])robability that Brachylepas was 

 an offshoot from that line, which by suppression of the 

 peduncle and modification of the lower valves of the capi- 

 tulum, accompanied by widening of the carina and rostrum, 

 had evolved into a sessile Cirripede. 



It is probable that the ancestral species of Pycnolepas 

 existed in the Upper Jurassic (Tithonian), for the two 

 recently-described species, Brachylevas (?) Jimbriatus and 

 B. (?) tithonicus (1912, Geol. Mag. pp. 505-508, pi. xxiii.), 

 each represented by a single carina from Stramberg, Moravia, 

 agree in every way with the structure of the carina in the 

 species of Pycnolepas. The relationship of those Stramberg 

 species to P. rigidus and P, fallax was pointed out at the 

 time, but, since the present evidence with regard to P. rigidus 

 and P. fallax was not then known, the two Stramberg 

 carinae were included provisionally in Brachylepas, to which 

 P. fallax had been referred by Dr. H. Woodward. 



When we compare Brachylepas* (text-fig. 5) and Pycno- 

 lepas (text-fig. 4) with the recent pedunculate Cirripede 

 Pollicipes mitella (text-fig. 2), we see that P. mitella has 

 precisely the same arrangement of the upper valves of the 

 capitulum. Brachylepas, however, is widely difierentiated 

 structurally by the presence of several whorls of imbricating 

 plates at the base of the capitulum, and in this cliaracter has 

 a close outward resemblance to the recent sessile Cirripede 

 Cafophragmus polymerus (text-fig. 3) of the subfamily 

 Chthamalinise. There is fairly strong evidence, both positive 

 and negative, to support the supposition that Pycnolepas has 

 a peduncle with large plates and no lower whorl of valves, 



* I have already discussed the relationship of this form with the 

 recent Cirripedes Catophrac/nms polymerus and Pollicipes mitella in 

 a former paper (see Geol. Maj^-. 1912, pp. 356-358). 



