REPORT ON THE RADIOLAEIA. 683 



are quite different. Whilst in the Tholonida the axes of the domes are dimensive axes, 

 and these are separated by aunuhir constrictions lying in diagonal planes, in the Zonarida 

 we find the contrary ; the axes of the domes are here diagonal axes, and these are 

 separated by annular constrictions lying in dimensive planes. However, this definition 

 agrees absolutely only in the four-chambered Zonarium and in the eight-chambered 

 Zonidium, whilst in the six chambered Zoniscus only four domes are disposed accoixling 

 to this law, two others, however, in the same manner as in the Tholonida. Therefore 

 this genus is intermediate between both families. 



The Cortical Shell of the Zonarida is in all eases completely latticed and of regular 

 lentelliptical fundamental form, as in the nearly allied Larnacida and Tholonida. The 

 three dimensive axes are constantly of different sizes, each with two equal poles ; 

 commonh" (as in the human body) the principal or longitudinal axis is the longest, the 

 sagittal (or dorso-ventral) axis the shortest ; the transverse (or lateral) axis being 

 intermediate between them. Of the three dimensive planes the lateral plane is the 

 largest (determined by the principal and transverse axes) ; the smallest is the equatorial 

 plane (crossed by the transverse and sagittal axes); the sagittal plane (determined by the 

 sagittal and principal axes) being intermediate between them. 



The annular constrictions of the cortical shell which produce the dome-shaped 

 protuberances are dift'erent in number in the three known genera — two, three, or four. 

 To each constriction often (but not always) corresponds an internal latticed septum, 

 which connects the cortical with the medullary shell. The number of the cupolas is 

 always double the number of the annular constrictions by which they are separated, 

 therefore four, six, or eight. 



In all known Zonarida the sagittal septum is cpiite constant, derived from the original 

 axial rod, which lies in the principal axis. By ramification of this axial beam and 

 reticular connection with the sagittal girdle arises the sagittal septum, which we found first 

 in Octopyle, halving the four gates of Tetrapyle. Whilst this sagittal septum (between 

 right and left halves of the body) is common to all three known genera of this family, the 

 number and shape of the other annular constrictions are different. In Zonarium (PL 50, 

 fig. 9) we find only one transverse constriction (in the equatorial plane), in Zoniscus 

 (fio-s. 10, il) two parallel transverse constrictions (parallel to the equatorial plane, on both 

 sides of it). Zonidium (fig. 12) is a combination of both foregoing genera; it has three 

 parallel transverse constrictions (one in the equatorial plane, and one on each side of it). 



The Latticed Domes (cupolas or chambers) of the cortical shell exhibit correspondingly 

 a different number and disposition in the three known genera. In Zonarium are found 

 only four crossed chambers, separated by the sagittal and transverse septa ; the axes of 

 the four crossed domes are diagonal axes, whilst in the similar Staurotholus they are 

 dimensive axes (principal and transverse axes). In Zonidium we find eight domes, each 

 cupola of Zonarium being halved by a diagonal septum. Zoniscus is intermediate 



