DESCRIPTION OF 27 



PLATE XXVI. 

 Fig. 1. AROH^EOCIDARIS AGASSIZI. 69^8 



1 a. A fragment preserving several plates of the body, with a small group 



of the spines, natural size. 

 1 b. A single plate enlarged. 

 1 c. Profile view of the same. 



1 d. A single spine enlarged to twice the natural size. 



Fig. 2. ARCH^EOCIDARIS KEOKUK. 699 



2 a. A single plate with the spine, as it occurs on the surface of the stone. 



The figure is very slightly enlarged. 



2 b. Profile view of a single plate enlarged. 



Fig. 3. ARCH^EOCIDARIS SHUMARDANA. 699 



3 a. A fragment, natural size, showirig several plates and a small group of 



the spines. The figure is a part of a much larger mass of plates and 



spines. 



3 6. A single plate enlarged. 

 3 c. Profile view of the same. 



3 d. A single spine twice enlarged. The accompanying figures are a section 



of the spine, and enlargement of the surface markings on the middle 

 and lower parts. 



Fig. 4. ARCH^IOCIDARIS WORTHENI. 700 



4 a. Upper side of a crushed specimen. The small imbricating plates of the 



summit are not well shown in the figure. 



4 b. Base of specimen, showing the displaced and crushed plates, with se- 

 veral broken spines. 



<|<I7 4 c. A single plate enlarged, showing the mamillary tubercle, the annula- 

 tion and surrounding disc, etc. 



4 d. Profile of the same. 



4 e. Enlargement of one of the ambulacral areas and the adjacent plates 

 in outline, showing the oval pores, etc. 



4 /. A single spine, natural size; length a little more than two and a 

 quarter inches, imperfect at the extremity, but restored in the figure. 



4 g. Enlargement of a portion of a spine near the base, showing the cre- 



nnlations on the edge of the anulation. 



Fig. 5. ARCH^EOCIDARIS NORWOODI. 701 



5 a. A fragment, showing several imperfect spines and a plate of the body. 

 5 b. A plate enlarged, from the range adjacent to the ambulacral area. 



:. c. Profile view <f the same. 



5 d. A spine nearly twice enlarged, showing the striate lovrer part, and the 



mnricate and spinulose character of the middle and upper part. 

 A farther enlargement oi 

 longitudinal stria;, which 

 the articulating surface. 



o e. A farther enlargement of the lower part of the spine, showing the 

 longitudinal stria 1 , which are strongly marked on the annulation above 



j'nht- 



