u.Ai!K.| PSAMMECTTINITS CINGULATUS. 55 



PSAiMlM ECHINUS Agassiz. 



PSAMMEOHTKUS CINGULATITS Clark. 



^ Plate XX, Figs. In-i. 



rsammechinu!^ c'nHiuIalns Clark, 18!U. Johns Hopkins ITiiivfrsity Circulars, No. 87, 

 p. 7(i. 



Deter mi native characters. — Test smiill, hemispherical, with circular 

 circuinfereiice; sides inflated; iqipcr suvfjice eleviited; base flattened 

 and concave. Ambulacra widi', with two rows (►f i>rimary tubercles, 

 sixteen or seventeen in each series; poriferous zones depressed, tri- 

 o-eminal. Tuteramlnilacra about one and one-half times the width of 

 the ambulacra; plates covered with numerous uncrenulated and imper- 

 forate tubercles. IVfouth opening large, with ten deep incisions. 



Dimensions. — Transverse diameter, J inch; heif^lit, ^ inch. 



Description. — This delicate form, of which two very perfect specimens 

 were examined by the writer,»is hemispherical in shape with circular 

 circumference, inflated sides, elevated upper surface, and flattened base. 

 In the \icinity of the mouth opening- the low^er surface is depressed, 

 producing a slight concavity. 



The ambulacral areas are wide and composed of eighteen to twenty 

 plates (PI. XX, Fig, 1/). The lower sixteen or seventeen of each series 

 bear ]m)minent tubercles that occur in a continuous line to the ])eris- 

 tome. On either side of the prinuiry tubercles are smaller ones of like 

 shape. The three pore pairs are arranged in semicircular form, the 

 two upper separated from the lower by one of the secondary tubercles 

 (PI. XX, Fig. Ir/). 



The interambulacral areas are about one and one-half times as wide 

 as the ambulacral an<l the plates about flfteen in number. Each ]>late 

 bears a large tubercle in the center, and irregularly arianged about it 

 smaller tubercles of various sizes (PI. x;x. Fig. le). 



The mouth opening is large, with ten deep incisions (PI. xx, Fig. Ih). 

 The ai)ical disk is lacking, but the broken opening is large and ])en- 

 tagonal in form. 



IxeJated forms. — This unique species is quite unlike anj- hitherto 

 described, and is readily characterized by the arrangement of the 

 tubercles and pore pairs. As the only rei)resentative of this genus 

 re(M)gnized in American dej^osits it has especial interest. 



Locality and f/eolof/ical horizon. — This species is labeled as simply 

 from New Jersey, but in character of jireservation is not unlike other 

 specimens from the yellow limestone of the middle marl bed (upper 

 Cretaceous) of Timber creek. 



Collection. — Philadelphia Academy of Natural Sciences. 



