OLIGOCENE ECHINODEKMATA. 165 



Order ATELOSTOMATA. 



Suborder ASTERNATA. 



Family NUCLEOLITID.E. 



Genus AMBLYPYGUS Agassiz. 



Amblypygus merrilli Twitchell, n. sp. 



Plate LXXVI, figures la-d; Plate LXXVII, figures la-b, 2a-b; Plate LXXV1II, figures la-b. 



Determinative characters. — Test large to very large, subhemispherical to depressed sub- 

 conical; circular or subcircular in marginal outline; sides rounded; under surface concave. Apex 

 central or subcentral. Ambulacral areas tumid, broad, widest between apex and ambitus; 

 petals wide open, nearly reaching the ambitus, poriferous zones wide, slightly depressed. Peri- 

 stome large, irregularly subtriangular, oblique, slightly excentric anteriorly. Periproct very- 

 large, larger than the peristome, subelliptical to subpyriform, longest longitudinally, about 

 midway between peristome and posterior margin. 



Dimensions. — Specimen A (type): Length 115.5 millimeters; width 114 inillimeters; height 

 62 millimeters. Specimen B : Length 73 millimeters; width 69 millimeters; height 40 millimeters. 



Description. — This superb Amblypygus, which is one of the largest of American echinoids, 

 -is the first representative of this rather rare genus to be reported from the United States. The 

 type specimen was collected by G. P. Merrill, of the United States National Museum, and 

 the species is therefore named in his honor. The test of the present species is large to very 

 large, ranging from about 70 millimeters to about 115 millimeters in diameter. It is sub- 

 hemispherical to depressed subcorneal in form, the upper surface declining immediately and 

 equally on all sides from the central or subcentral apex; circular or subcircular in marginal 

 outline. The margin is rounded and inflated; the under surface concave centrally. 



The ambulacral areas are broad, widest about midway between apex and ambitus, thence 

 narrowing to the apex and peristome; they are slightly tumid, thus forming five tumid ridges 

 from apex to peristome. The dorsal portions of the areas are subpetaloid, the petals being 

 long, about reaching the ambitus, wide open at their extremities. The poriferous zones are 

 wide, slightly depressed; both outer and inner rows of pores round, pairs of pores conjugated by 

 long, narrow grooves. 



The interambulacral areas are broad. The surface of the whole test, including the inter- 

 poriferous areas, is covered with numerous small perforate tubercles, with sunken scrobicules. 



The apical system is large, central or subcentral. Most of the area of the system is occu- 

 pied by the large, slightly tumid, granulated madreporite, which appears to be the enlarged 

 right anterior basal plate. There are four large genital pores, the two anterior being nearer 

 together than the posterior pair. The radial plates and their perforations can not be discerned 

 on the specimen. 



The peristome is very large, irregularly subpyriform, slightly excentric anteriorly. It is 

 placed in an oblique position, its longest diameter being about 30° to the transverse axis of the 

 test. 



The periproct is very large, larger than the peristome, subelliptical to subpyriform, situated 

 about halfway between the peristome and posterior margin, though slightly nearer the peri- 

 stome; its longer diameter extending longitudinally. 



Related forms. — Amblypygus merrilli is not closely related to any other eclmioid from the 

 United States. Of foreign forms it resembles both A. americanus Michelin from the Tertiary of 

 Jamaica ' and A. alius Duncan and Sladen from the Tertiary of India, being intermediate between 

 them in relative height. The proportion of the mean diameter to the height in A. merrilli is 

 1:55, whereas in the Jamaican form, according to Duncan and Sladen, it is 1:409 and in the 



1 The only good description of A. americanus Michelin is given by Duncan and Sladen in The Tertiary Echinoidea of Kachh and Kattywar, 

 p. 13, which contains additions to Desor's descrii. ion based on a specimen in the British Museum. So far as the writer has been able to ascertain 

 the form has never been figured. 



