REPORT ON THE RADIOL ARIA. 615 



The flxmily L a r n a c i d a immediately follows the Larcarida as the next simple 

 group of all Larcoidea; some genera of both groups (such as Larnacalpis and 

 Coccolarcus, or Larnacantha and Larcidium) may easily be confounded from their 

 being so much alike. In both the lentelliptical shell is composed of two concentric 

 shells, an inner (medullary) and an outer (cortical) shell. But the connection between 

 these shells and the construction of the inner shell is cpiite different in the two groups. 

 Whilst in the Larcarida the medullary shell is connected with the cortical shell simply 

 by radial beams, here in the Larnacida this connection is effected by two latticed 

 lamellee, which are the lateral wings of a transverse girdle. Therefore we encounter 

 here for the first time that peculiar mode of growth which characterises the greater 

 part of the L a r e o i d e a, but particularly the Pylonida. But whilst in the Pylonida 

 lietween the three crossed lattice-girdles, remain four open gates, here in the Larnacida 

 the gates become closed by lattice-work; the lentelliptical cortical shell becomes perfect. 



The most simple genus of Larnacida, and no doul)t the common ancestral form of 

 this whole family, is Larnacilla (PI. 50, figs. 1, la, \h). The most important shell 

 of this typical genus is composed of a simple lentelliptical medullary shell and of 

 three elliptical latticed girdles surrounding it, perpendicular one to another. These 

 three " dimensive girdles " lie in the perimeter of the three dimensive planes, the minor 

 (and first) in the equatorial plane, the second (and major) in the lateral plane, the third 

 (and intermediate) in the sagittal plane. Therefore we have before us the same 

 "trizonal shell" as in the important genus Trizonium among the Pylonida. But whilst 

 in Trizonium, as in all Pylonida, the four gates between the guxlles remain open, here 

 in Larnacilla they become perfectly closed by lattice-work. 



Tlie formation of the typical "XarnaciYZa-shell" begins with a simple, spherical or 

 lentelliptical lattice-shell, from both sides of which arise two latticed "lateral wings" 

 opposite on the poles of the transverse axis. These two wings are comparable to the 

 lateral chambers of Tliolartus (among the Tholonida), but differ by two large 

 openings. Each wing is a short cylindrical tube with latticed wall, open at both ends ; 

 the axis of the tube (going through the centre of the open ends) is parallel to the 

 principal axis of the whole shell (and of the central chamber). Therefore l)oth wings 

 form together a transvexse ring, the middle of which encloses the central chamber. 

 The distal parts of both wings grow towards the poles of the principal axis ; if they 

 became united here, the second (lateral) girdle would be complete. Between it and the 

 first girdle four open gates remain (" Tetrapyle ") ; Ijut these become overgrown l)y the 

 third or sagittal girdle, and at last the gates between this and the two other girdles 

 become closed by lattice-work. This perfect fenestration of the trizonal cortical shell, 

 and the comjijlete closing of the gates between the girdles by network, is the ouly- 

 difterence between Trizonium and Larnacilla. 



In TjCtrnacilla and in the nearly allied Larnacidium (only differing by radial spines 



