1188 THE VOYAGE OF H.M.S. CHALLENGER. 



10. Cyrtocalpis gromia, n. sji. (PI. 51, fig. 11). 



Shell rough, ovate, very thick-walled, with siibregular, circular pores of somewhat variable size. 

 The diameter of the internal, simple, subspherical cavity is five to six times as gi-eat as the thickness 

 of the wall Mouth tubular, cylindrical, very narrow, only one-sixth as broad as the shell. 



Dimensions. — Shell 0'16 long, O'll broad; mouth 0'02 broad. 



Habitat. — Western Tropical Pacific, Station 225, depth 4475 fathoms. 



Genus 530. Mitrocalpis,^ Haeckel, 1881, Prodromus, p. 427. 



Definition. — A r c h i c o r i d a with a double ovate or ellipsoidal shell, composed of 

 two parallel lattice-plates, which are connected by numerous l)eams. Apex without horn. 



The genus Mitrocalpis and the following closely allied Spongocfrtis differ from the 

 four preceding genera of Archicorida in the complicated structure of the shell-wall, 

 which in the former is double, in the latter spongy. The mouth is in both genera 

 constricted, narrow, and the apex without a horn. Mitrocalpis has been derived 

 probably from Cyrtocalpis in the same way as Peripijrariiis from Bathropyramis, by 

 development of spines on the surface of the shell, which at equal distances from the 

 latter become connected by branches, thus forming an outer envelojje. 



1. Mitrocalpis palliata, n. sp. 



Shell ellipsoidal, large, one and a third times as long as broad. Inner shell very thin-walled, with 

 irregular, polygonal pores of very variable size and form. From its nodal points there arise numerous 

 bristle-shaped radial beams, about as long as the diameter of the mouth, wliich are connected at equal 

 distances from the surface by delicate threads, forming the outer enveloping shell. The surface of 

 the latter is smooth. The thin bars of the inner shell are twice to four times as broad as those of 

 the outer. Mouth about one-seventh as broad as the shell, with a short tubular peristome. 



Dinunsions. — Shell 0'35 long, 0"27 broad ; mouth 0"04, distance of both shells 0'037. 



Habitat. — Central Pacific, Station 271, surface. 



Genus 531. Spongocyrtis,^ Dunikowski. 

 SjMiu/oci/rtis, Dunikowski, 1882, Denkschr. d. k. AkaJ. d. Wiss. Wien., vol. xlv. p. 31. 



Definition. — A r c h i c o r i d a with a spongy ovate shell, the wall of which is 

 composed of irregular spongy wickerwork. Apex without horn. 



The genus Spongocyrtis differs from most other Monocyrtida in the spongy 

 structure of the shell, which is found in only very few^ other genera of C y r t o i d e a, 

 e.g., in Spongopyramis and Spongomelissa. It has been derived either from the similar 



' Mitrocalpis = 'V\a\>a,n basket ; fiir^a, xety^Tis. - Spongocyrtis =S]pongy basket; aioyyi, xv^ti;. 



