THE SESSILE BARNACLES. 329 



PACHYLASMA GIGANTEUM (Phllippl). 



1854. Pachylasma yujanti'inn, Philippi, DARWIN, Monograph, p. 477, pi. 19, 

 figs. 5a-5d. 



Cat. No. 12060. Strait of Messina. Seguenza, in Jeffrey's collection. 



PACHYLASMA DARWINIANUM Pllsbry. 



1912. Pachylasma darwinianum PILSBRY, Proc. U. S. Nat. Mus., vol. 42, p. 

 293. 



Type. Cat. No. 43465, U.S.N.M., from Albatross Station 5168, 

 Tawi Tawi group of the Sulu Archipelago, 80 fathoms. 



PACHYLASMA CHINENSE Pilsbry. 

 1912. Pacliylasma cMnense PILSBRY, Proc. U. S. Nat. Mus., vol. 42, p. 293. 



Type. Cat. No. 43471, U.S.N.M., from Albatross Station 5301, 

 China Sea, near Hongkong, 208 fathoms. 



PACHYLASMA CRINOIDOPHILUM Pilsbry. 



1911. Pacliylasma crinoidophUum PILSBRY, Bull. Bureau of Fisheries, vol. 

 29, p. 81, fig. 11 ; pi. 17, figs. 1-11. 



Type. Cat. No. 38675, U.S.N.M., from Albatross Station 4934, 

 off Kagoshima Gulf, in 152 fathoms. 



Genus HEXELASMA Hoek. 





 1913. Hexelasma HOEK, Stfeo^a-Expeditie, Monographie 31fr, pp. 157, 214. 



Hoek described this genus as follows : 



Compartments six ; carina, carinolateral and lateral compartments with 

 alee, but without radii, the rostrum having neither radii nor alre. Parietes not 

 porous and without longitudinal ribs on their inner surfaces. Basis mem- 

 branous. Opercular valves subtriangular. Mouth with the labrum not notched 

 in the middle; mandibles with 4 to 5 sharply pointed teeth; nuixillne with 

 numerous spines beneath the notch. Third pair of cirri resembling more 

 those of the fourth than of the second pair. No caudal appendages. Species 

 living in deep water. 



By the texture of the compartments and valves, the absence of 

 radii and the absence of an adductor ridge in the scutum, Ilexelasma 

 resembles Pachylasma; but it differs by having no trace whatever of 

 caudal appendages, and in the typical species by having a mem- 

 branous basis. In H. americanum and H. callistoderma the basis is 

 calcareous, but very thin in the center, so that in specimens removed 

 from the support the very thin, filmlike central region adheres 

 partly to the support, so that I formerly described callistoderma as 

 having the central part of the basis membranous. There is an in- 

 flected rim at the bases of the compartments very much thicker than 

 the true basis. 



