186 BULLETIN 88, UNITED STATES NATIONAL MUSEUM. 



URASTERELLA GIRVANENSIS, new species. 

 Plate 28, fig. 5. 



Tetraster, sp. ind., Nicholson and Etheridge, Mon. Silurian Foss. GLrvan Dist., 



Ayrshire, fasc. 3, 1880, p. 325, pi. 21, figs. 9, 10. 

 ? Eoactis simplex Spencer, Mon. Brit. Pal. Asterozoa, pt. 1 (Palseontogr. Soc. for 



1913), February, 1914, p. 30, pi. 1, fig. 4. 

 Urasterella girvanensis Schuchert, Fossilium Catalogus, Aidmalia, pt. 3, April, 



1914, p. 44. 



Description hy N. and E. — "Body stellate, with slowly tapering 

 arms, wliich increase little in width throughout their whole length [the 

 distal ends being unknown]; upper surface unknown, but the body 

 was convex and probably granular. [The pieces of the disk are seen 

 through the mouth and consist of many tiny plates arranged in more 

 or less regular circles about the slightly larger central ossicle.] Ave- 

 nues of the arms broad, and not bridged over by any of the plates [this 

 is a shp of the pen, for the ridged ambulacralia he horizontal and 

 deep in the grooves and are directly opposite one another; the podial 

 openings are fairly large and in the usual position laterally]; marginal 

 ambulacra] plates [ = adambulacrals] moderately convex, trans- 

 versely elongated, or oblong in form, and less in mdth than the 

 ambulacral plates; those in the angles of the rays are the largest, and 

 somewhat more elongated tlian the others; oral plates not visible." 

 The interbrachial areas are structurally very interesting because in 

 each one there is a single narrow but long axillary interbrachial 

 ossicle that distaily margins the axil, and in front of this is a minute 

 pair of adambulacral oral armature pieces. On either side of the 

 axillary interbracliial are the most prominent adambulacrals, about 

 five in number, being larger than the other distal ones, wliich are 

 considerably narrower and of fairly uniform size throughout the rays. 



Formation and locality. — A single specimen from the Upper Ordo- 

 vicic at Thraive Glen, Girvan, Scotland. A good wax squeeze of the 

 holotype was furnished by Doctor Bather. The original is in the col- 

 lection of Mrs. Robert Gray, at Edinburgh, Scotland. 



Remarlcs. — This species is clearly a Urasterella, a fact which was 

 also noted by Nicholson and Etheridge, as they state: ''In some of 

 its characters it approaclies very closely to T. (?) asperriinus, Salter, 

 sp." In its interbrachial skeleton ZL girvanensis retains youthful 

 generic characters, seen in the weU-developed interbrachial axillary 

 ossicles. After the above was written appeared Spencer's paper 

 above cited. Before the writer was aware of this work he had pro- 

 posed the name Z7. girvanensis. If we are treating of the same 

 species Spencer's name takes precedence. 



Cat. No. 60611, U.S.N.M. 



