138 



BRITISH PALAEOZOIC ASTEROZOA. 



tudinal ridge which is especially prominent in the proximal region of the arm. It 

 is probable that this ridge is a paxilla-shaft. 



On each side of the radialia is one broad row of adradialia which alternate 

 with that series and are distinctly six-sided. A stout broad paxilla-shaft extends 

 from each of these. There are at least two other rows of adradialia on each half 

 of the arm, and possibly there may be a third row just at the base. These outer 

 adradialia are all distinctly smaller than the first row, and appear to have longer 

 and thinner paxilla-shafts. The extra length of the spine may be only apparent and 

 due to the fact that the ossicles are slightly displaced and lie on their sides so that 

 they are seen partially in profile view. At any rate, the appearance presented is 

 very characteristic not only of this but of related species (compare also Text- 

 fig. 92, p. 143). 



X. Re 



Text-fig. 88. — Plan of ossicles on the apical surface of Urasterella thraivensis. Lettering as in 



Text-fig. 92, p. 143. x 6. 



A distinct row of infero-marginalia borders the arm. The sutures between 

 these ossicles run at an angle to a line measured across the breadth of the arm. 

 Frequently the ossicles look as if they overlap. All these rows of ossicles persist 

 throughout 37 mm. of the length of the arm, but distally the differentiation 

 between the ossicles becomes less obvious and the paxilla-shafts less pronounced. 

 The structure of a considerable portion of the distal end of the arm cannot be 

 made out. 



The odontophor is not visible in apical view. The disc immediately over the 

 odontophor is covered by one or two small paxillas which occupy the interradial 

 angles. 



Oral Surface (Plate IX, fig. 2; Plate X, fig. 1; Text-fig. 89).— The best oral 

 views are shown by casts from D. 254. An account of the structure of the 

 mouth-parts is given (pp. 133-4) and of the adambulacralia and ambulacralia 

 (pp. 134-5). 



Arm in Gross-section. — The cross-section of the arm must have looked some- 

 what similar to that figured for U. pulchella (Text-fig. 80, p. 128). The apical 



