MESOPAL^ASTER COMPLICATUS. 87 



The mouth-angle plates are stout and triangular. They are succeeded by 

 adambulacralia, which meet across the groove and completely hide the ambu- 

 lacralia. There are two adambulacralia to each infero- marginal except at the 

 extreme tip. 



Ornament. — The ornament is of a rather coarse pustulose type. 



Measurements. — R : r : : 6 mm. : 2*25 mm. Width of arm at base is 2"1 mm. 



Horizon and Localltij. — Upper Ordovician (Ashgillian) of Girvan, Ayrshire, 

 Scotland. 



2. Mesopalaeaster complicatus, n. sp. Plate II, fig. 9 ; Plate III, fig. 5 ; Text- 

 figs. 46, 47, 48. 



Material. — Four specimens are known. Two are on one slab (D. 74), which 

 has a counterpart. The third (D. loo) shows only an imprint of the apical 

 surface. These are in the collection of Mrs. Gray, and are from the Starfish bed 

 of Thraive Glen. The holotype is that nearest the margin of the slab possessing 

 two specimens (D. 74). A fourtli specimen from the same locality is in the 

 collection of the British Museum (Nat. Hist.). It is registered as E. 13122, and 

 shows the oral surface only. 



Specif c Characters. — Forms somewhat more advanced than M. priimis. The 

 arms are more prolonged, the ossicles of the disc less distinct, the adradialia more 

 prominent, and the intermarginalia better developed. There are also more 

 adambidacralia in proportion to the infero-marginalia, and the mouth-parts are 

 more complicated. 



Apical Surface (Plate II, fig. 9 ; Text-fig. 46). — Neither the imprints on 

 D. 74 nor that on D. 133 give complete casts of the apical aspect. A restoration 

 can only be obtained by collation of the details presented by the three specimens. 

 The central portion of the disc is, as usual, sunk because of post-mortem change, 

 and the ossicles are in consequence somewhat disarranged. It is not quite clear 

 whether there are two or three rings of intermediate plates round the centrale. 

 The restoration (Text-fig. 46) shows two rings, and represents the appearance 

 of the cast from D. 133. On the other hand, the cast from the holotype on 

 D. 74 suggests that there may have been three rings of somewhat smaller ossicles 

 in this specimen. The ossicles themselves are somewhat irregular in size and 

 shape, and there is a strong suggestion that they were but loosely embedded in 

 the covering skin. 



The primary interradialia are of approximately the same shape as those of 

 M. primus, but they are somewhat reduced in size. The primary radialia are large 

 and, just as in M. primus, look as if they were the second member of this series. 

 In two of the radii of D. 133 the primary radial is unusually large and distinctly 

 tumid, with a stellate base. In other arms this ossicle is not so prominent. The 



