58 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



and a vacant space which may be an anal opening. Five other 

 small quadrangular plates are intercalated, one proximally to each 

 of the large radial basal plates. Outside of these 12 intercalary 

 plates there is a ring of 10 quite large and conspicuous plates, 5 of 

 which are radial and 5 interradial in position. The radial plates are 

 largest and are the basal pieces of 5 medial or radial columns, while 

 against the 5 interradial plates rest the 10 inframarginal columns. 



Madreporite abactinal, quite small, situated immediately above a 

 large interradial plate and between two basal supramarginal plates. 

 Its position in relation to the anal opening is apparently as in P. 

 incomptus. 



Inframarginal plates in mature examples about 14 to a col- 

 umn, and when not displaced completely, inclosing the adarnbu- 

 lacral plates and margining the rays. The axils are occupied by a 

 very large, oval, axillary marginal plate. Each inframarginal bears 

 upon its outer surface one to three short, slender, pointed spines, 

 which are finely striated longitudinally. 



Adambulacral plates small, subquadangular in outline and about 

 19 in a column in specimens having 13 inframarginals. Each plate 

 bears on both the outer and inner edges a single spine and is like 

 those of the inframarginal ranges. 



The oral armature consists of 5 pairs of elongate adambulacrals, 

 each pair being situated directly orad of the large axillary marginal. 



Ambulacral plates I -shaped, practically opposite or but very 

 slightly alternating, one to each adambulacral plate. In the ex- 

 panded medial portion the ambulacralia are slightly superposed one 

 above the other proximally, while the lateral portion of each plate 

 is very slender, leaving between adjoining pieces comparatively large 

 podial openings. 



Locality and formation. From the Trenton limestone, as follows: 

 The type-specimen was found in the " shelly layers" at Trenton 

 Falls, New York (Amer. Mus. Nat. Hist., No. 199), and another 

 specimen was secured there by Mr. Rust (New York State collection). 

 Dr. Walcott collected a slab with three individuals in the Lower 

 Trenton at Rathbone Brook, near Newport, Herkimer County, New 

 York; and Mr. Taylor got two specimens at Deerfield, Oneida County, 

 in the same State (Mus. Comp. Zool., Nos. 26 and 3). Mr. W. R. 

 Billings found a specimen in the Trenton at Lachine, Quebec, and 

 another from Government House Bay, Ottawa. 



Remarks. H. matutinus is intermediate in structure between the 

 older E. narrawayi and the younger H. incomptus. It is larger than 

 the former, has more plates in each of the columns, while the ambu- 

 lacrals are deeply cut out for the podia, which is not the case in H. 

 narrawayi. As H. matutinus is most closely related to H. incomptus 

 more detailed comparisons must be made between them. Actinally 



