/. Dinocystis Barrom. 



V. 545 



what would correspond to the opening of the cap. Thus the under 

 surface is hollowed, but since the central region is always filled 

 by matrix one cannot ascertain its structure in full (PI. XXI, 

 Figs, lb, 2b). If the fossil be placed on a flat surface, the actinal 

 pole reaches a height of 14 mm. (1), 9 mm. (2), 9*5 mm. (4 and 6), 

 6 mm. (7). But, owing to the concavity of the under surface, the 

 thickest portion of the actual specimen would be slightly less and 

 would lie about half-way between the actinal pole and the periphery. 

 (Diagram 2.) 



DlAGKAMS OF DiNOCYSTIS. 



(1) The upper or actinal surface. 



(2) Vertical section across the middle of the test (based chiefly on 3 and 7). 



0, the actinal pole ; Ami, the five grooves radiating therefrom and passing over 

 the periphery ; As, anus ; /, frame of abactinal surface ; imb, imbricated 

 peripheral area of same ; m, thin membrane of central area of same. 



On the Upper Surface, from the actinal pole, five Radial Grooves 

 pass outwards over the test, each curving as it goes in a sinistral 

 or contra-solar direction (Diagram 1). Each reaches the periphery, 

 and passes along it for fully a fifth of the circumference, or may 

 even pass over to the under surface, so that in nearly all the 

 specimens one or more of the grooves is visible from below. 



Apart from the contortion of the specimens, the grooves are not 

 wholly dominated by pentamerous symmetry. They show the 

 primitive division into one anterior, and two pairs of lateral 

 grooves, and are besides not quite regular in their course. 



The grooves, as seen on the internal casts, are clearly marked 

 channels, deeper and narrower towards the periphery, where they 

 have undergone more compression. They are bordered on each 

 side by a row of equidistant small knobs, which become smaller 

 distalwards ; the knobs are either opposite each other or slightly 

 alternating, and each pair is connected by a faint ridge (Figs, lc 

 and 2d). A wax squeeze shows that this appearance is due to the 

 former presence of plates flooring the groove (Figs. 2e and /). It 

 is probable that these plates originally were in two alternating rows, 

 but that they usually came to lie in pairs without alternation. They 

 may even have fused (as was perhaps also the case in Jlaplocystis 

 Roemer) ; at any rate there is no sign of a median suture. Between 

 successive plates, on each side, was a pore, represented in the 



