The Structure of Dinichthys 



133 



in any other animals. This circumstance must be remembered in comparing this figure 

 with the Arthrodiran fragments. 



Median-Basal. — This plate (Text-figures 12, 13, 14, 15 MB, and 16) is the most 

 massive one of the whole head roof. It is 9 cm. long, 15 cm. broad and 5 cm. thick;" and 

 forms the central part of the posterior consolidated arch of the roof with its transverse 

 commissure (tc) and double sockets (ds). In contradistinction to most of the other plates, 

 MB can quite often be found isolated. The limits between it and its neighboring plates 

 are well developed. At the sides MB overlaps EB. In front it is overlapped by both 

 C plates. 



Externo-Basal. — This plate (Text-figures 12, 13, 14 and 15 EB) forms the wings of 

 the posterior consolidated arch. Along its posterior margin run the half-round thickenings 

 (ht) on the ends of which the joint processes are placed (Jp) . The upper, thinner part of EB 



forms the largest part of the bottom of 

 the postero- lateral impression (PL). EB 

 is overlapped by C, and partly over- 

 lapped by M. However, the limits be- 

 tween these three plates are difficult to 

 see from the inside. 



On the outside of EB we find two 

 branches of the sensory canals (Text- 

 figure 12) which form a V-shaped figure. 

 The point of this goes close to the joint 

 socket, with which it is connected by a 

 short canal (VII). One of the branches 

 extends over on the M (VI). The other 

 goes nearly to the edge of C (VIll). (In 

 some specimens this branch runs over 

 onto C). 



Marginal. — This plate (Text-fig- 

 ures 12, 13, 14 and 15 M) is a solid long 

 one, which together with EB and PM 

 forms the postero-lateral wing of the head 

 roof. Nearly along its center runs the 

 posterior high ridge of the lateral head 

 thickening (Rd). It forms the side wall 

 Text-figure 16. of the postero-lateral impression (PL). 



median-basal plate of Dinichthys interrnedius Xhe M overlaps the PtO and C On the 



Nwb.: a, from inside; b, from outside. „ ,^ .-j^ ^^„ r j ^l^ „^„ „,,, .,„ i 



^ - ,,,,,,.„ r outside are round the sensory canals. 



C position or central; ds, double sockets; EB, position or externo- 



basal; tc, transversal commissure. They make a threc-branched figure, very 



The 



'These measurements were made from a small example of D. mtermedms (No. 7891) in the American Museum. 



