MESOPAL^ASTER PRIMUS. 



85 



specimens, however, are small, and tlie specimens of the closely related species 

 M. complicatus are also small. Together they form cumulative evidence that the 

 British " true Mesopalaeaster " were smaller and not so highly ornamented as their 

 contemporaneous American brothers. (Compare also the account of the British 

 species of Promopalseaster, p. 92.) 



Apical Surface (Plate II, fig. 2; Text-fig. 44). — D. 70 is the only specimen 

 which shows an imprint of the apical aspect of this species, but fortunately it 

 yields very clear casts, which give the structure in almost complete detail. The 

 central portion of the disc sunk somewhat during post-mortem change. Neverthe- 



44 



45 



Text-fig. 44 (on left). — Plan of the surface of the disc and one arm of Metopalceasler primus (based 



on D. 70). X 12. 

 EX T-PiG. 45 (on right). — Plan of the oral surface of one arm and the mouth-region of Mesopalmasler primus 



(based on D. 70c-) x 12. 



i, the ossicles are but little disturbed. A distinct centrale is seen, surrounded 

 a circlet of seven plates. It is not clear that any of these plates were connected 

 with an anal opening. The remainder of the sunk area of the disc is filled in by 

 a second circle of small secondary plates. 



Large primary interradialia are present. The proximal edge of these plates is 

 convex, with a median nose. The distal edge is concave, and the two side edges 

 slope together slightly in a distal direction. I have not been able to identify 

 a madreporite with any certainty. 



Between each interradial is situate a small radial. Just behind the small 

 radial is a large primary radial. This is broader than long, and distinctly breast- 

 plate-shaped. The remaining radialia are longer than broad, and alternate with 

 the supero-marginalia throughout the greater part of the length of the arm. 



