616 



THE VOYAGE OF H.M.S. CHALLENGER. 



on the surface) the " trizonal shell " is an external or " cortical shell," enclosing the 

 central capsule, the interior of which only contains the simple central chamljer and the 

 jointed proximal parts of both lateral wings. In the other genera of Larnacida (by 

 proceeding growth) this trizonal Larnacilla-shell becomes enclosed by the growing- 

 central capsule and is now only a " medullary shell," whilst on the outside of the central 

 capsule in the same manner is developed an outer cortical shell [Larnacalpis, 

 Larnacantha) ; and perhaps the same process may be repeated. But sometimes also 

 this cortical shell becomes doubled by a simple envelop of network (Larnacoma). 

 In the Larnacospongida the cortical shell is composed of a spongy framework (corre- 

 sponding to the Spongolarcida in the foregoing family). 



The lattice-work of the Larnacida is commonl)^ irregular (as in most other 

 L a r c o i d e a), and its pores have little signification for the different species. On the 

 outer surface often arise radial spines, symmetrically disposed either in dimensive planes 

 or in diagonal planes. 



The central capsule is constantly a true lentellipsis or a "triaxial ellipsoid," 

 characterised by three halving, elliptical dimensive planes, perpendicular one to another 

 It bears a different relation to the skeleton in the two subfamilies of Larnacida. In 

 the first subfamily, the LarnaciUida {LarnaciUa, Larnacidium), the central capsule 

 encloses only the simple spherical or lentelliptical central chamber ("simple medullary 

 shell"), and is enveloped hy the trizonal cortical shell. In the other subfamily, the 

 Larnacalpida [Larnacalpis, Larnacospongus, &c.), this trizonal " Larnacilla-s]\e]\" 

 becomes enclosed by the overgrowing central capsule, which now becomes enveloped by 

 an external, latticed or spongy, lentelliptical " cortical shell." 



Synopsis of the Genera of Larnacida. 



I. Subfamily 



LarnaciUida. 

 Medullary shell simple, .spherical or suhspherical. 

 Cortical shell lentelliptical, trizomxl; between them 

 four gates. 



Shell without radial spines, 272. LarnarJUa. 



Shell with radial spines. 



II. Subfamily 



Larnacalpida. 



Medullary shell Larna- 

 fi'ZZa-shaped, double; 

 the inner spherical or 

 suhspherical, the outer 

 lentelliptical and tri- 

 zonal. 



Cortical shell simple or 

 double, but not spongy. 



Cortical 



shell -! 

 simple. 



Without radial 

 spines, 



With radial 

 spines. 



Cortical shell double, with- 

 out radial spines, 



Cortical .shell simple or i Shell without radial spines, 

 double, wholly or partly ■! 

 spongy. ( Shell with radial spines, . 



273. Larnacidium. 



274. Lurnucalpis. 



275. Larnacantha. 



276. Larnacoma. 



277. Larnacosponijm. 



278. Larnacostvpa. 



