CHELONOBIA CARETTA. 395 



facing each other exactly ; and this holds good in specimens attached 

 separately, without any apparent cause for this want of symmetry. 

 The largest specimen which I have seen was 2*1 in basal diameter; 

 and this, which was a steeply conical individual, was 1*1 in height; 

 and therefore nearly twice as high as an ordinary large specimen of 

 C. testudinaria- 



Structure of Shell and Radii. — The descending sheath and radiating 

 septa are of very variable thickness, and have their basal edges finely 

 dentated. The septa are not continuous, from the circumference to the 

 sheath, in unbroken plates, but are irregularly divided into separate, 

 often short portions, and even occasionally into mere points. The 

 sheath differs from that of the other two species in having loopholes 

 for the entrance of ribbons of corium onlv on the eight lines of suture, 

 and not (with rare exceptions) in the middle of each compartment : 

 this is evidently due to fewer filaments of corium being here sufficient 

 to supply the less deep interspaces between the radiating septa ; for in 

 this species there are no flattened cavities or tubes running far up the 

 shell. The inner lamina of the walls cannot be here distinguished, for 

 a solid, flat, calcareous surface extends from the circumference, between 

 the radiating septa, to the sheath. The sheath, had it not been from 

 the light thrown on this part by the other species, would have certainly 

 been mistaken for the inner lamina of the walls. The absence of the 

 flattened cavities or tubes extending up the parietes, seems to be the 

 least varying character; and serves to distinguish this species from 

 those worn and massive specimens of C. testudinaria, which have 

 narrow and not-notched radii. 



In specimens in which the radii are not developed, no vestige of the 

 outer lamina can be detected, the lateral faces of the adjoining 

 compartments being closely pressed together ; but in specimens in 

 which the radii have been developed and have grown, the outer lamina 

 of course is present, and is extremely thick, with the growing edge 

 having a branched and mammilated surface, as in C. testudinaria, but 

 with the external surface not pitted or notched as in that species. 

 The inner portion of the radius, whether or not developed, has nearly 

 the same appearance, consisting of rather thick transverse septa, 

 branching from a central ridge, which is sometimes obscure. 



The Ales are remarkably little prominent, as least in those specimens 

 in which the radii are not developed, so that Spengler seems to have 

 thought that the structure of the shell was essentially different from that 

 of Balanus, which certainly is by no means the case. The edges of the 

 alee are very thick, nearly as thick as the inner portion of the radius. 



The Opercular Valves hardly present any essential difference, com- 

 pared with those of the other species ; but the occludent margin of 

 the scutum is apt to be more sinuous, and its rostral end blunter and 

 squarer. The carinal end of the tergum is also squarer than in any 

 common variety of C. testudinaria ; the external furrow or spur, near 

 the carinal margin, is very indistinct, and even sometimes is quite 

 absent. 



Neither the mouth nor cirri present any deviations from the generic 

 character. 



