226 BRITISH PALEOZOIC ASTEROZOA. 



for the fitting of the first adambulacral is also shown. It is rather larger in the 

 drawing than in reality because of the slight distortion of the cast. The great 

 general resemblance between this mouth-angle plate and the same plate in a recent 

 Asteroid has already been remarked (p. 196). 



The first ambnlacral has moved a little from its exact fitting on to the mouth- 

 angle plate. The fitting shows that a peg-and-socket joint ensures a firm fitting 

 of the two ossicles. 



The mouth-angle plates in full oral view (Text-fig. 162) present a very 

 characteristic appearance. They are much swollen, and divided almost throughout 

 their length by a distinct deep triangular cleft bordered by two thin ornamented 

 ridges. This same drawing also shows the interradial ossicles. One long ossicle, 

 which may represent the odontophor, occupies the whole space between the second 

 adambulacralia of neighbouring arms. Proximal to this ossicle are two smaller 

 ossicles, and behind it four ossicles of various sizes. All these ossicles have swollen 

 surfaces and appear to imbricate in the direction of the mouth. There appears to 

 be some variation on this arrangement in the various specimens, possibly due to 

 the degree of compression to which the form has been subjected. It is evident 

 that increased compression would result in an apparent increase of the interradial 

 areas due to a forcing downwards of a portion of the apical surface. 



The madreporite is very large. As shown in Text-fig. 159, p. 222, it may 

 overlie other interradial plates. The plate figured has slightly split in its anterior 

 region, and the split shows that the plate is very thin. It is possible that it was 

 also slightly flexible. Specimen no. 25349 shows the madreporite sticking close to 

 the curve of the interradial region. The direction of the grooves on the madreporite 

 suggests that the opening of the pore canal was at the edge of the disc — that is, in 

 the primitive Ophiuroid position (see pp. 37, 181). 



Apical Surface (PI. XV, figs. 5, 6; Text-figs. 157, 158, 163, 164).— The 

 arrangement of the ossicles is seen very clearly in casts from two impressions in the 

 Sedgwick Museum (48 and a/517 A). The plan given (Text-fig. 157, p. 221) is 

 drawn from these two specimens. It is strongly reminiscent of that met with in 

 Mesopalseaster primus (Text-fig. 46, p. 86). The centre of the disc is occupied 

 by a number of small plates, arranged approximately in two circles. Outside 

 this is a circle of ten plates, five primary radialia and five primary interradialia. 

 The primary interradialia are a little larger than the primary radialia. As stated 

 above, the madreporite is not associated with a primary interradial as in the true 

 Asteroidea, but is oral in position. Distal (and oral) to the primary interradialia 

 are a number of plates (Ir.) occupying the same position as do the intermarginalia 

 in Mesopahmster. The radialia are hexagonal plates alternating in the proximal 

 region of the arm with the supero-marginalia, and separated from these latter plates 

 at the very base of the arm by a few adradialia. Just below the supero-marginalia, 

 and except m compressed specimens only showing in side view, are two rows of 



