84 BRITISH PALAEOZOIC ASTEROZOA. 



liowevcr, appear to ine to be somewhat shorter than in tlie holotype of that species, 

 but there is no certainty that any of the arms is preserved throughout its entire 

 length. Photographs of two of the fragments are given in PL II, fig. 8, and PL 

 III, tig. 7. 



The specimen (E. J 3531) is from the Lower Bala rocks, in a lane to Gelli Grin 

 Farmhouse, 2^ miles south-east of Bala (see p. 75). 



Genus MESOPAL^ ASTER, Schuchert emend. 



Palseaster (part) of Authors. 

 1868. ? Arciaster, Hall, Twentieth Eep. N.Y. State Cab. Nat. Hist., p. 287 ; rev. ed., 1868-1870, p. 329. 



1914. Mesopalasaster, Schuchert, Fossilium Catalogus, Animalia, pt. 3, p. 24. 



1915. „ Schuchert, Bull. 88, U.S. Nat. Mus., p. 74. 



Oeneric Gharacters. — The odontophor well enclosed by the succeeding infero- 

 marginalia. Plates of apical area often swollen and nodose. In the Ordovician 

 species the proximal supero-marginalia help to form the boundary wall of the disc, 

 but in the Silurian species they may be pushed iuAvards as in the Promo- 

 palaeasterinae and the Xenasterida3. Madreporite imkuow^n. 



The genoholotype is M. sltaferi from the LTpper Ordovician of America. As 

 already mentioned, the genus probably contains several lineages and will be split 

 up when the forms are known in greater detail. The British species M. prirnus 

 and M. compJicatus are very similar to M. sliaferi (for list of species of genus 

 see p. 98). 



1. Mesopalseaster primus, n. sp. Plate II, fig. 2; Plate III, fig. 8; Text-figs. 



44, 45. 



Material. — Imprints of two specimens are known, one (D. 70 and D. 70f) con- 

 sisting of both imprint and counterpart, the other (D. 172) an imprint of the oral 

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

 Thraive Glen. 



Specific Characters. — Central portion of disc covered with two distinct circlets 

 of plates. Primary radialia and interradialia large and distinct. A single row 

 of small adradialia. Adambulacralia about twice as numerous as the infero- 

 marginalia. 



The species in many respects approximates to M. sha.fferi, Hall (compare the 

 figures given with Schuchert's figures, 85, pi. 8, figs. 1 and 2). The infero- 

 marginalia, however, of the American species carry each a strong spine, and the 

 two species differ in size. If I had only found one specimen of the British species 

 I should have been inclined to place it as a young form of M. sliaferi. Both 



