CIRRIPEDIA. 



499 



As regards the geological distribution of the Balanidce, the oldest 

 known types are the Protobalanus and Palceocreusia of the Devonian 

 rocks of North America. The former of these is a minute Balanoid 

 which possessed the unique peculiarity that the shell consists of no 

 less than twelve compartments (the " rostrum," the " carina," and 

 five pairs of " lateralia "). In the genus Palceocreusia, again, the 

 shell was embedded in the substance of the corallum of Favosites, 

 and the compartments of the shell are fused to form a single un- 

 divided plate, covering a tubular sub-cylindrical basis. 



With the above-mentioned exceptions, no Palaeozoic representa- 

 tives of the Balanoids have hitherto been recognised, the Balanus 

 carbonarius of the Carboniferous rocks of Saxony being, according 

 to Zittel, not a Cirripede at all. In the Mesozoic rocks, moreover, 

 no undoubted representative of the Balanoids is known till the 

 Upper Chalk is reached, a species of Chthamalus appearing in this 

 formation. The fossil described by Seeley as a Balanoid from the 



356. — Balanus coficavus, of the natural size, from the Pliocene Tertiary (Crag), a, Side- 

 view of the shell ; b, Tergum ; c, Scutum. (After Darwin — copied from Zittel.) 



Lias, under the name of Zoocaflsa, does not appear to be properly 

 referable to this group. In the Tertiary rocks, the remains of 

 Balanoids are not uncommon, the recent genus Balanus (fig. 356) 

 occurring in deposits as old as the Eocene, and being still more 

 abundantly represented in the Oligocene, Miocene, and Pliocene. 

 In the later Tertiaries the group is further represented by forms 

 belonging to the genera Chthamalus, Acasta, Pyrgoma, and Coronula, 

 all of which are known by living types. 



The remaining family of the Sessile Cirripedes is that of the 

 Verrucidtz, comprising only the single genus Verruca. In many 

 respects the Verrucidce, approach the Bala?iidce, but the shell is 

 composed of six valves only, and is unsymmetrical, whilst the scuta 

 and terga (forming the operculum), though movable, are not fur- 

 nished with a depressor muscle. The Verrucidce appear, so far as 



