FROM THE WHITE CHALK. 841 



the matrix of this Urchin has, however, raised doubts in my mind as to the accuracy of 

 the stratum and locahty. Mr. Walker it appears bought it from a dealer in Warminster, 

 who may have had it in exchange along with some foreign specimens. Catopygus pyri- 

 formis is found in the Senonian of Maestricht, Ciply, and Sens, Yonne, and is a very 

 characteristic Urchin of this stage of the Cretaceous formations. 



The following description and figures of a new species of Pseudodiadema have been 

 forwarded to me by ray valued friend, the Rev. Professor Thos. Wiltshire, F.G.S., as a 

 contribution to my Addenda. 



Pseudodiadema fragile, Wiltshire. PI. LXXX, figs. 1 — 11. 



" Test somewhat small, thin, circular, depressed, almost fiat on upper surface, slightly 

 convex on under. Poriferous zones narrow, straight, pores unigeminal on upper surface, 

 trigeminal on lower. Ambulacra narrow above, rather wider below, with two rows of 

 small perforated tubercles from oral aperture to above ambitus. Interambulacrai wide, 

 two rows of sm.all primary perforated tubercles in the middle extending the whole dis- 

 tance, and two rows of secondary perforated tubercles on lower side, ceasing after passing 

 the ambitus. Miliary zone granular below, showing a few secondary tubercles above 

 and below the ambitus. Mouthropening large, peristome circular, lobed. Discal 

 opening large, pentangular. Spines small, slender, straight ; surface striated, striae 

 interrupted and diverging, and producing a roughened surface. 



Dimensions. — Height less than | inch, transverse diameter 1^ inches. 



Description. — The test is thin, circular, almost fiat on upper surface, slightly rounded 

 on under. In the specimen from which the drawings were made the upper and under 

 sides, prior to their fossilisation, had been subjected to considerable pressure, and brought 

 into closer contact than they were in the living stage. The distance between the upper 

 and the under sides therefore cannot be accurately determined ; probably the figured 

 specimen was under half an inch in height. 



The ambulacral areas are straight and narrow (about two-ninths the width of the 

 ambulacra at the upper side, and nearly one-half at the peristome), contracting in their 

 range upwards. Two rows of about nine perforated tubercles (one tubercle to each 

 plate) start from the under side, and cease soon after passing the ambitus. The tubercles 

 are small, placed in small areolae, with slightly projecting bosses and crenulated summits, 

 and are separated by an undulating series of granulations. The poriferous zones are 

 narrow and straight. The pores are oval and simple, and are arranged in single file 

 throughout the zones, save near the peristome, where they form into a few transverse 

 lines of sets of three pores. 



The interambulacrai areas are four times the width of the ambulacral at the equator, 



44 



