118 BRITISH PALAEOZOIC ASTEROZOA. 



Arms xi and xii give the best view of the large and subtriangular mouth-angle 

 plates (PI. VII, fig. 3). The interradial side of the triangle is thick and looks 

 like a stout bar. It lies parallel with and adjoining the corresponding bar of 

 the neighbouring mouth-angle plate. The proximal side of this pair of bars 

 is excavated so as to form a deep hollow bordered by a thick lip. The torus fits 

 against the lip. Rows of spine-pits run across the torus. The spines themselves 

 have usually fallen away, but a few of them may be seen in the interradius 

 containing the madreporite. The inner side of each mouth-angle plate is hollowed 

 out very much as is the jaw of Lajnvortliura (compare with PI. T, fig. 10). It is 



Tkxt-fig. 71. — Plan of the ossicles on the oral surface of Lepidaster grayi. Ad., adambulacralia ; I.M., 

 infero-marginalia ; M., madreporite ; M. P., month-angle plates ; T., torus ; V., ventro-lateralia. x 3. 



not possible to see the articulation of the first ambulacralia with the mouth-angle 

 plates. 



The adambulacralia in the proximal third of the arm are somewhat smaller 

 than those immediately distal to the disc, and are considerably modified. A wash 

 drawing is given (PL VII, fig. 2), which shows the modifications assumed by the 

 adambulacralia. The three proximal adambulacralia of the figure each show a 

 distinct nose on their inner side. This inner nose end is considerably thickened, 

 and the thickening extends as a gradually thinning ridge across the ossicle. The 

 fourth and fifth adambulacralia are turned somewhat on one side, but they show 

 that the ridge meets a corresponding projection from a neighbouring infero- 

 marginal. As we follow the adambulacralia distal wards the cross-ridge widens 



