70 BRITISH PALEOZOIC ASTEROZOA. 



Genus GIRVANASTER, novum. 



Generic Characters. — The most proximal supero- and infero-marginalia large 

 and high, producing a swollen disc; arms short and stumpy. Primary radials 

 umbrella-shaped, primary interradials small. Adambulacralia many more than 

 the infero-marginalia. 



The only known species is Girvanaster scidptus, n. sp., of which moulds are 

 found very abundantly in the Upper Ordovician (Ashgillian) of Thraive Grlen, 

 Ayrshire. 



1. Girvanaster sculptus, n. sp. Plate II, fig. 7 ; Plate III, fig. 6 ; Text-figs. 34, 35. 



Material. — The collection of Mrs. Gray contains more than thirty specimens, 

 many of them possessing counterparts. Very good casts can be obtained from the 

 moulds, and it is thus possible to give a full account of the form. The specimens 

 figured, PI. II, fig. 7, and PL III, fig. 6, are taken as the cotypes of the species. 

 Moulds showing the oral surface of two individuals, and the apical surface of a 

 third, also occur on a slab from the same horizon in the collection of J. Wright, 

 1886. 



Apical Surface (Plate II, fig. 7; Text-fig. 34). — The central portion of the 

 disc sank somewhat after death, but the ossicles have only been slightly dis- 

 placed. It will be seen from the description that the arrangement of the ossicles 

 is fundamentally of a very primitive nature, although individual ossicles have been 

 modified somewhat profoundly. There is a small centrale surrounded by a circlet 

 of six plates, all somewhat flat and ornamented. Outside these, again, is a circlet 

 which must represent the five primary radialia and five primary interradialia. 

 These, however, have been considerably modified, and differ much from the 

 appearance usually presented. 



The primary radialia are flat, and shaped very much as the silhouette of an open 

 umbrella with a stout handle. The top of the umbrella and the median portion of 

 the handle are stoutly calcified. It is this portion of the plate which is shown 

 in the plan as figured. There are, however, lateral extensions from the handle 

 which are much less stoutly calcified, and which fit on to the edges of the neigh- 

 bouring supero-marginalia. The flexible extensions would allow the dorsal 

 surface of . the disc to rise and fall, a movement possibly connected with 

 respiration. 



The primary interradialia are flat, and not nearly so large as is usual amongst 

 the Hudsonasteridge. There is no trace of a special madreporiform plate, or 

 of a space in which such a plate could have been lodged. Possibly one of the 

 primary interradialia was perforated to serve as a madreporite. The second radial 

 is separated from the primary plate by inner prolongations from the first pair of 



