346 BALANID^E. 



generally large. Walls either thin and smooth, or thick 

 and plicated longitudinally. Colours various, pale purple, 

 greenish, white, and, in E. plicatus, owing to the exposure 

 of an intermediate lamina of shell, bright orange-yellow. 

 Radii, either of considerable width, with their summits 

 oblique and rounded, as in the first two species of the 

 genus, or very narrow, as in the last two species. Elminius 

 plicatus is the largest species, and is sometimes one inch in 

 basal diameter. The outer surface of this latter species is 

 occasionally much corroded. 



Scuta : these are of the usual shape ; in E. Kingii and 

 modestus there is no adductor ridge and no crests for the 

 depressor muscles; in E. plicatus and simplex, on the other 

 hand, there is a well developed adductor ridge and crests 

 for the lateral depressor muscles ; in some individuals, also, 

 of E. plicatus there are small crests for the rostral depressores. 



The Terga are remarkable for their variability in all the 

 species \ in many specimens of E. Kingii and modestus the 

 basal margin on the carinal side of the spur is deeply hol- 

 lowed out. The width and acumination of the spur varies 

 in all the species. In E. plicatus and simplex this valve is 

 remarkably like that of Tetraclitaporosa. In some specimens 

 of E. Kingii the terga and scuta are firmly calcified together. 



Structure of the Parietes and Radii. — As in Tetraclita, 

 the two lateral compartments are necessarily broad. The 

 parietes are never porose, but consist, in appearance, of a 

 single layer of shell. In E. modestus the basal internal edges 

 of the parietes are smooth, but in the other species they are 

 striated longitudinally with short ridges, or sometimes with 

 sub-cylindrical projections. In those specimens of E. 

 plicatus, which have externally suffered much corrosion, the 

 walls have been rendered extremely thick, by the inward 

 production of these ridges or plates ; and in this case the 

 ridges are not confined to the basal edges, but extend 

 upwards close to the sheath. The basal surfaces of the walls 

 in these latter specimens resemble those of Chelonobia, but 

 the walls in that genus have an internal lamina, which here 

 is not the case. The radii are wide in E. Kingii, and of mo- 

 derate width in E. modestus, with their summits oblique and 

 smoothly rounded, and their sutural edges not in the least 



