472 NORTH AMERICAN INDEX FOSSILS. 



XV. COMAROCYSTITES BillillgS. 



Calyx ovate or pyriform, with three basal plates ; above these 

 are 8 to ii cycles of mostly hexagonal plates in irregular order. 

 Ambulacrum at apex, short, straight, unbranched ; from each end 

 rises a pair of uniserial arms, bearing long, cylindrical pinnules. 

 Anal area near apex composed of a pyramid of five pieces and an 

 outer circle of about five plates. Column round, smooth; its 

 plates very thin. Ordovicic. 



27. C. punctatus Billings. Ordovicic. 



Length of calyx about one and one half inches ; diameter one 

 inch. Each plate with a deep depression occupying nearly its 

 entire area, and usually with thin, erect lamellae at right angles to 

 the sides of the plates ; these lamellae are occasionally crossed by 

 others, parallel with the sides of the plates. (Type of genus.) 



Trenton of Ottawa. 



XVI. Agelacrinus Vanuxem. 



Calyx in the form of a depressed or convex disc, without 

 column, and attached by the entire under surface; composed of 

 numerous small, polygonal, sometimes imbricating plates which are 

 perforated by fine pores which, for the most part, are united in 

 pairs. Mouth central, surrounded by four oral plates ; radiating 

 from this are five small, more or less curved ambulacra, which 

 are protected by a double row of covering plates. Anus eccen- 

 tric, provided with a valvular pyramid. Ordovicic-Mississippic. 



2S. A. (Lepidodiscus) cincinnatiensis Roemer. (Fig. 1784, ^.) 



Ordovicic. 



Circular, depressed-convex above. Ambulacra sunk nearly to 

 the level of the disc; four sinistral and one dextral. Disc plates 

 imbricating. Usually found on Rafinesquina ahernata. 



Cincinnatian of Cincinnati region. 



29. A. hamiltonensis Vanuxem. Devonic. 

 Disc plates sculptured, not imbricating. 



Hamilton of New York. 



30. A. (Lepidodiscus) alleganius Clarke. Devonic. 

 Disc plates imbricating. Ambulacra very slender and all point- 

 ing dextrally. 



Chemung of New York and Pennsylvania. 



