EEPOET ON THE RADIOL ARIA. 



861 



figs. 6—9), is this number multiplied ; the two opposite primary apophyses are here 

 crossed by numerous perpendicular lateral branches, and these are again united by 

 secondary perpendicular ramules which are parallel to the apophj^ses themselves ; there- 

 fore each plate here forms a rhombic shield pierced by very numerous (often more than 

 one hundred) small quadrangular or circular pores. Only two of these numerous parmal 

 pores are the primary aspinal pores (sometimes, as in PI. 136, fig. 8, distinguished by 

 their larger size); all others are secondary or coronal pores. Therefore Phatnaspis rejjeats 

 the characteristic formation of Coscinaspis, from which it differs in its ellipsoidal shell. 



Moreover, the four other genera of the ellipsoidal Belonaspida correspond perfectly to 

 certain genera of the spherical Dorataspida, and may be derived from these by hyper- 

 troph)^ or stronger development of two opposite equatorial spines. In Thoracaspis and 

 Belonaspis (PI. 139, figs. 8, 9) the surface of the shell-plates is without crests and 

 dimples (in the former without, in the latter with accessory by-spines) ; they thus corre- 

 spond to Dorataspis and Diporaspis among the Doratasj)ida. In two other genera the 

 surface of the shell-plates bears a network of elevated crests, separating funnel-shaped 

 dimples, in DictijaspHs Mdthout, in Coleaspis with by-spines ; these correspond to 

 Ceriaspis and Hystrichaspis 2imoi\gi\iQ Dorataspida. The small by-spines, covering the 

 surface, exhibit the same forms as in the Dorataspida ; l)ut they are usually smaller, 

 and less developed in the Belonaspida. 



The twenty radial spines are commonly more or less compressed or two-edged, 

 sometimes very flat, broad, and triangular (PI. 139, figs. 8, 9). In some species (mainly 

 of Phatnaspis) they are very long, thin, and needle-shaped (Monogr. d. Radiol., 1862, 

 Taf. xxii. figs. 10—12). Rarely they are cylindrical, four-edged, or prismatic. The 

 Belonaspida testify by this and other peculiarities to their near relation to the Di])ora- 

 spida and Phractacauthida and their older origin from Zi/gacantha. 



The Central Capsule is in the Belonaspida ellipsoidal, and more or less prolonged 

 iu the hydrotomical or longitudinal axis of the shell. It is constantly smaller than the 

 enveloping ellipsoidal shell, and separated from it by the calymma. Its structure 

 and the shape of its nucleus are the same as iu the nearly allied Dorataspida. 



Synopsis of the Genera of Belonaspida 



I. Subfamily 



Coleaspida. 

 Forty parmal pores (two 

 aspinal pores on each plate 

 no coronal pores). 



Plates not dimpled, without ] ■' ^ ' 



prominent crests. 'j ^^-.^j^ by-spines, 



-r,, , ,. , , .,, , ( Xo by-spines, 



Plates dmipled, with a net- I •' '^ 



^york of prominent crests, j ^^,.^j^ by-spines, 



II. Subfamily 



Phatnaspida. 

 Eighty to two thousand or more parmal pores (two aspinal 



pores on each plate, surrounded by two to one hundred 



or more coronal pores). 



No by-spines (plates not 

 dimpled). 



."i71. Tliiiracaspis. 



•372. Bi:loiiai<j)is. 



37.'?. Dicti/aspis. 



37 1. Vi>lfas]))H. 



37.5. l'liiitna»jns. 



