THE SESSILE BARNACLES. 263 



Compartments generally extremely thick, six, but the rostrum is 

 composed of three very intimately united compartments, the sutures 

 visible internally and in worn specimens also on the outside. 

 Parietes porous (septate), but sometimes filled np solidl}'^ nearly to 

 the bases of the septa. Basis flat, membranous; opercular valves 

 narrow, not nearly closing the orifice, the scuta and terga united by 

 a chitinous articular ligament (pi. 62, fig. 6). Labrum having a long 

 row of teeth and a median notch in the margin. Third pair of cirri 

 long, but structurally like the second; later cirri long, having seg- 

 ments with tAvo pairs of spines, with little tufts of fine spines between. 



Type. — C. testudlnaria (Linnieus). 



Dhtributhn. — Tropical and temperate seas, on turtles, crabs, and 

 manatees. Pliocene of Grand Canary and Italy. 



This group retains a primitive character in the incompletely con- 

 crescent rostrum and rostro-lateral compartments. Tlie sutin-es nuiy 

 be traced on the inside and in worn individuals externally. It is 

 a transition from the octomerous stage to the hexamerous. In other 

 respects it is rather highly specialized. In the least modified species, 

 C. iKitulo., the structure of the wall reminds one of BaJaims, Ijut I 

 venture the opinion that the resemblance is probably due to con- 

 vergence, the structures being homoplastic. The more specialized 

 species are admirably adapted to the rough conditions of existence 

 on the backs of sea turtles, the walls being enormously thickened and 

 the stature low. T\\Qy have a superficial resemblance to the whale 

 barnacles, Coronula. 



C. testud'inana is attached superficially, and when removed the 

 turtle shell is found quite smooth and uninjured. It is the same 

 Avitli C. yatula on crabs. In C. caretta the growing peripheral edge 

 cuts iuto the turtle shell, which commonly covers or curves up over 

 the edges of tlie barnacle. Darwin quite appropriately compares 

 this penetrating power of the barnacle to that of growing roots. 



Chelonobta patella (Eanzani)^ is evidently a pen error for C. 



The following fossil species have been described : C. hermsphAjerlca 

 Ivothpletz and Simonelli,^ Pliocene of Grand Canary; C. emisphv- 

 rlca De Alessandri, 190G. C. depressa Seguenza, 1875, Pliocene of 

 Sicily. C. capellinii De Alessandri,^ Miocene and Pliocene, Italy. 



All of them are of the testudlnaria type, and therefore afford no 

 information on the phylogeny of the genus. The recent C. patula 

 is the most primitive of the known species in structure. 



1 Zoolo>riJ::chor Anzoiger, vol. .".S, p. 4G1, Novemljer, I'Jll. 



2 Zeitschrift deut.sch. Gcol. Oes., vol. 42, 1891, p. 724, pi. 36, lig. 2. 



3 Palaeontograpbica Italica, vol, 12, 1006, p. 313. 



