804 



THE VOYAGE OF H.M.S. CHALLENGER. 



spine) are also partly Cladoph racta (the Staurasiiida, PI. 137, figs. 5-8), partly 

 Peltophracta (tlie Lycliuaspida, Pis. 135-136). The differences and relations of 

 these tribes are placed synoptically in the following table : — 



All Dorataspida are trne Icosacantha, and the twenty spines, composing the spherical 

 shell, are equally developed, regularly disposed according to the IMiillerian law, and of 

 equal size ; also the distance of their plates from the common centre is equal. Nevertheless 

 they are never of perfectly the same form ; in consequence of their peculiar disposition 

 in five zones (each with, four spines) certain slight differences are effected, so that with 

 accurate knowledge of the peculiar shell-composition it is generally not difficult to 

 distinguish the spines of the equatorial, the two tropical, and the two polar zones. 



Already the central bases, by which the twenty spines are united in the centre of 

 the sphere, exhibit certain differences in the five zones. Commonly these bases are small 

 pyramids, all meeting wdth their apex in the centre, and the triangular faces of the 

 neighbouring pyramids are supported one upon another. The four equatorial pyramids 

 are commonly six-sided, the other sixteen five-sided ; but sometimes there are eight 

 six-sided and twelve five-sided basal pyramids ; two opposite polar spines on each pole 

 having a six-sided base (like the four equatorial), the other two polar spines on each 

 pole having a five-sided base (like the eight tropical). Earely the central bases are 

 perfectly grown together, forming a single spherical central piece of acanthin. 



The three different fundamental forms of radial spines, which are found in all 

 AcANTHARiA, the cylindrical, the two-edged, and the four-edged (spines with circular, 

 with elliptical, and with square transverse section respectively) occur also in the 

 difi"erent groups of Dorataspida ; but commonly the two-edged or compressed form is 

 prevalent in the Diporaspida, the four-edged or quadrangular form in the Tessaraspida. 

 In the majority of species the spines are thickened in the shell-face, where the 

 apophyses arise, and thinner towards the tw^o ends. Usually the outer or distal 

 part of the spine (outside the shell) is longer than the inner or proximal part (inside 



