454 ADDITIONAL NOTES 



descends in a zigzag line down the middle of the area, and the zonal border of the plates 

 has a single row of the same sized granules, which separates the areolas from the poriferous 

 zones, and forms a series of crescents throughout the area (fig. 2 c). 



The apical disc is large (fig. 2 a), and placed rather behind the vertex of the test ; 

 the ovarial plates are narrow (fig. 2 d), and the duct-holes perforated near the apices 

 of the plates ; the madreporiform body occupies, as usual, the right antero-lateral plate, 

 which is the largest ; the three anterior ocular plates rest upon the ovarial plates, with 

 which they alternate, whilst the two posterior oculars are placed between the two postero- 

 lateral and single ovarial plates (fig. 2 d) \ the vent-opening is very large. 



The base is flat, and the mouth-opening wide ; the peristome is divided into ten 

 nearly equal-sized lobes; all the tubercles in this region are small, and the poriferous zones 

 at the base of the ambulacral areas are very much crowded, the pores being arranged in 

 oblique rows. 



Affinities and differences. — This species diflers so much in its general physiognomy from 

 the typical forms of its congeners that it requires a careful examination to be satisfied that 

 it is a Hemicidaris, the size and number of the semi-tubercles, extending as they do so 

 high up the area, and the small and uniform magnitude of the primary tubercles, produce 

 so many rows of tubercles on the flanks of this urchin (fig. 2 b, d), that it might readily 

 be referred to the genus Acrocidaris rather than to the group to which it belongs. In 

 Acrocidaris, however, the tubercles are very unequal in magnitude on the sides of the test, 

 and each ovarial plate supports on its centre a small, primary, perforated and crenulated 

 tubercle, a character which is quite diagnostic of this genus. The greater width of the 

 ambulacra, and the presence of a double row of semi-tubercles, extending three parts 

 up the sides, distinguish this species from its congeners ; the smallness of the primary 

 tubercles in the inter-ambulacra, and the increased number and nearly uniform size of the 

 same throughout the rows, distinguish it likewise from Hemicidaris Bavidsoni, Wr., another 

 Portland species, with which it has many afiinities. 



Localiiy and Stratigrapliical position. — This urchin was discovered in the Portland 

 Oolite, at Brill, and was purchased from the person who collected it therefrom by my 

 friend, the Rev. P. B. Brodie, who kindly communicated the specimen for this work ; the 

 test is rather distorted, and much concealed by a small, encrusting oyster. At the same 

 locality my friend collected the large Ecldnohrissus Brodiei, Wr., of which I have given a 

 figure of the natural size in PI. XLIII, fig. 3. 



