CIRRIPEDIA. 337 



fellow along one margin (the " occludent " margin), and 

 articulates with one of the terga along the opposite margin. 

 Similarly, each terguin opens and shuts against its fellow 

 along one margin (the " carinal " margin), and articulates 

 with one of the scuta along the opposite margin. The apex 

 of the terga (fig. 197, E) often forms a prominent beak, and 

 the basal margin is furnished with a process or " spur." The 

 scuta and terga are not only movable, but are furnished with 

 proper depressor muscles. 



As regards the distribution of the Balanidce in time, the 

 oldest known representative of the family, so far as is cer- 

 tainly known, has been indicated by Mr Seeley as occurring 

 in the Lias, and has been made the type of a new genus 

 under the name of Zoocapsa. So far as is known, no member 

 of the group occurs in any Palaeozoic deposit ; and negative 

 evidence is in. this case of considerable value, as the Balani 

 possess a shell which is readily preserved, whilst they adhere 

 to all sorts of marine bodies. With the above-mentioned 

 exception (which may, perhaps, be referred to the Verrucidce), 

 no fossil Balanoid has hitherto been discovered in sediments 

 older than the commencement of the Tertiary period. The 

 genus Balanus is the earliest of the group, and appears under 

 several specific forms in the Eocene rocks. In the Miocene 

 and Pliocene deposits, the Balanidce are abundantly repre- 

 sented by Balanus itself, and in the latter by the genera 

 Acasta, Pyrgoma, and Coronula. 



The remaining family of the Sessile Cirripedes is that of 

 the Verrucidce, comprising only the single genus Verruca. 

 In many respects the Verrucidce approach the Balanidce, but 

 the shell is composed of six valves only, and is unsymmet- 

 rical, whilst the scuta and terga (forming the operculum), 

 though movable, are not furnished with a depressor muscle. 

 The Verrucidce appear, so far as is known, to have com- 

 menced their existence towards the close of the Secondary 

 period, the Chalk having yielded one species. Verruca, 

 Stromia is found in the Coralline and Red Crags (Pliocene), 

 in Glacial deposits, and in existing seas. 



The third family of the Cirripedia Thoracica is that of the 

 Lepadidm or Pedunculated Cirripedes, commonly known as 



VOL I. Y 



