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



without annular constrictions (Larcarida, Larnacida, Pylonida) ; these form the subsec- 

 tion Pylolarcida. Two other families (Tholonida and Zonarida) are distinguished by 

 annular constrictions, which divide the regular lentelliptical shell into a number of dome- 

 shaped chambers or cupolas ; we call these Thololarcida. A third group, Spirolarcida, 

 comprises the Larcoidea with spiral growth ; the two families of Lithelida and 

 Streblemida. Finally, a fourth group, the Sorolarcida, is formed by the Larcoidea 

 with irregular shells, also two families, the Phorticida and Soreumida. 



The first family of Larcoidea, the Larcarida, contains the most simple forms, 

 beginning with Cenolarcus, a quite simple lentelliptical latticed shell. In Coccolarcus 

 we find already two concentric shells, connected by radial beams, an inner medullary 

 and an outer cortical shell. In Spongolarcus the lentelliptical shell becomes spongy. 



The second family, Larnacida, is very similar to the Larcarida, and seems to diverge 

 only by the different mode of connection between the two concentric lentelliptical shells. 

 But in truth this slight difference is of great morphological importance, as it depends on 

 a quite different and peculiar mode of growth. In the foregoing Larcarida (Coccolarcus, 

 &c.), the concentric shells originate in the same manner as in the concentric 

 Prunoidea and Sphaeroidea, by radial beams, which arise from the surface of 

 the inner (medullary) shell and become connected by a network to form the outer 

 (cortical) shell. Here, in the Larnacida, a quite similar shell originates in a quite 

 different way, first arrived at in the Pylonida (Trizonium). Both concentric shells 

 become here connected by peculiar lattice girdles, which are developed in the perimeter 

 of the three elliptical dimensive planes. Firstly, on both sides of a simple, spherical, 

 or lentelliptical central chamber, arise two lateral wings (on the poles of the transverse 

 axis), and build around the former a transverse girdle. This is crossed by a larger 

 lateral girdle, the minor axis of which is the major of the former, and perpendicular to 

 both girdles is yet developed a third, the sagittal girdle. If the open fissures or 

 " gates " between these three girdles become closed by network, we obtain Larnacilla, 

 the probable ancestral form of all Larnacida. 



Whilst in Larnacilla and Larnacidium this typical trizonal lentelliptical shell con- 

 stitutes by itself alone the whole skeleton, in the other Larnacida it becomes overgrown 

 by outer envelops, and so becomes enclosed in the interior of the central capsule as a 

 "Larnacilla-shsiped medullary shell." If the enclosing external envelops be simply latticed, 

 we get the subfamily Lariiacalpida; if they be spongy, we get the Larnacospoiigida. 



The third family, Pylonida, is the most important of all Larcoidea, as not only 

 the largest and most interesting number of species belong to it, but also many other 

 genera (far the greater part of all Larcoidea) may be derived from it. The peculiar 

 character of the Pylonida is determined by the imperfect fenestration of the lentelliptical 

 shell growing in the three dimensive axes in a quite different manner. Each elliptical 

 dimensive plane becomes circumscribed by an elliptical latticed girdle (or fenestrated 



