REPORT ON THE RADIOL ARIA. 631 



complete. R. Hertwig supposes that the minor axis of this sagittal girdle is 

 coustantly at the same time the major axis of the lateral girdle, but this is not 

 always the case. Very often the size of both these girdles is nearly the same, or one is 

 not much larger than the other. In this case the principal axis of the body is the 

 major axis of the second as well as of the third girdle. 



The characteristic " gates " of the Pylonida, or the large wide openings in their 

 cortical shell, remaining between the crossed latticed girdles, are in general roundish, 

 sometimes nearly circular, commonly more elliptical, kidney-shaped or semilunar, their 

 special form varying much according to the different form of the girdles. The 

 narrowest part of each girdle, or its " isthmus," in the case of the halves of the 

 transverse girdle is commonly at their origin from the medullary shell, in the case of the 

 halves of the lateral girdle at the poles of the principal axis, and in the case of the halves 

 of the sagittal girdle at the poles of the sagittal axis. The number of the gates is quite 

 constant in the difierent genera. If only one girdle (the transverse) be developed, we 

 find only two large gates, between the two wings on the poles of the principal axis 

 (in Monozonium, Amphipyle, Amphipylonium). In all other cases there are four 

 gates (determining the original name " Tetrapyle "), as well if only two or if all three 

 girdles be completed. If two girdles be complete (in Dizonium, Tetrapyle, Tetra- 

 pylonmm) the four gates lie opposite in pairs on the sagittal faces (two anterior and 

 two j)osterior gates), and are limited by the transverse and lateral girdles. If all 

 three girdles be complete (in Trizonium, Pylon in m, Pylozonium) the four gates lie 

 opposite in pairs on the lateral faces (two dorsal and two ventral gates), and are limited 

 by the sagittal and the transverse girdles. If we turn the shell through an angle of 

 90°, we have the same aspect as in the former group. A sagittal septum sometimes 

 becomes developed, beginning with two polar l^eams, rising from the poles of the 

 principal axis. If these polar beams become branched and connected with the middle 

 part of the lateral girdle, we get a latticed vertical septum, wliich divides the four gates 

 of Tetrapyle into eight gates, Octop>yle. 



The lattice-work of the Pylonida is commonly very variable and irregular, with 

 roundish meshes of very unec_[ual size, therefore without value in the determination 

 of the species. Commonly the outside of the shell is thorny, and often distinguished 

 by larger radial spines, symmetricallj'' disposed. We can separate these into two groups ; 

 " dimensive " spines, lying in one of the three dimensive axes (principal, transverse, or 

 sagittal), and " diagonal " spines, lying crossed in pairs in diagonal axes. Among 

 these latter eight diagonal wing-spines, which arise from the lateral edges of the four 

 gates, are particularly remarkable ; they are not only characteristic of Tetrapyle 

 octacantha, but also of a large number of other Pylonida, and form the starting-point 

 for many specific forms. 



The shell of the Pylonida is characterised by extraordinary variability and great 



