16(3 C. D. WALCOTT — DISCOVERY OF SILURIAN VERTEBRATES. 



strosa, except that it is open below and gives rise on the sides to what 

 appears to have been the support of the ribs. Further description will 

 be given in a final paper. 



The principal material upon which the genus and species are founded 

 is illustrated on plate 3. Figure 1 is a side view of a portion of a rather 

 large sheath. It shows the close transverse rings and the projecting 

 lateral rib sockets or supports. Figure 2 is a view from above of a por- 

 tion of the -heath shown in figure 1 to display the form and arrangement 

 of the lateral rib sockets or supports. Figure 3 is an enlargement of the 

 surface of a chorda] sheath to showthe characteristic network formed by 

 the crossing of the two series of elevated, raised, curved striae. It is con- 

 sidered that these represent the fibres of the sheath, while the vertical 

 rings shown in figures 1 and 5 are the calcified rings. The fusion of the 

 rings and the oblique fibers give rise to the continuous calcified sheath, 

 as in Chvniasra monstrosa. Figure 4" is a transverse outline of the chordal 

 sheath to show- that is was not closed on the ventral surface, ami figure 

 46 is a transverse outline cutting across the lateral extensions or rib sup- 

 ports. Figure 5 represents a portion of a small chordal sheath, showing 

 its flexible nature and indicating that the larger fish must have attained 

 considerable size. 



GAN01DEA. 



Si B-ORDER PlACODERMATA. 



Family Asterolepidida I f). 



ASTRASPIS DESIDERATA, X. si'. 



This type is represented by fragments of plates allied to those of 

 Asterolepis ornatus of the Devonian. 



The material upon which the species is founded is illustrated on plates 

 3 and 4. On plate 1 figure 6 shows the inner surface of a plate with a 

 portion broken away so as to exhibit the base and transverse sections of 

 the tubercles of the outer surface, and figure 7 represents the interior sur- 

 face of a plate for comparison with figure ('>. Figure 8 represents a frag- 

 ment of a supposed ventral plate of the body, figure 9 a plate referred to 

 the cephalic region, and figure If' a small elevated tuberculated plate. 

 Figure 11 shows the supposed inner surface of a plate similar to that 

 represented in figure 10. and figure 12 the inner surface of a plate similar 

 to that seen in figure 11. Figure 13 is a transverse section of a narrow, 

 elongate plate, showing a cellular structure and the projecting tubercles. 

 The latter expand at the summit into a round knob, the upper surface of 

 which is cut hv radiating stria', so as to give it a star-like Astrse-form 



