208 BRITISH PALAEOZOIC ASTEROZOA. 



plate is shown in the drawing. It follows from this that the plates marked by 

 Mr. Spencer as A } and Ad l were really A 2 and Ad,, and that the true proximal 

 ambulacral and its corresponding adambulacral had not yet fused to form a 

 mouth-angle plate." 



I have introduced a new drawing (Text-fig. 148) which shows the mouth-angle 

 plate in one of the interradii mentioned by Dr. Bather as showing the plate in 

 two pieces. It will be seen that the plate is in one piece and has precisely 

 the same structure as the mouth-angle plate of the recent Astropect&n figured 

 in Text-fig. 137, p. 195. The suture which Dr. Bather thought that he 

 could distinguish separating two elements is obviously the groove for the nerve- 

 ring. Dr. Bather allows me to say that he accepts this explanation. 



Measurements. — R : r : : 7 mm. : 3'1 mm. 



Horizon and Locality. — Upper Silurian (Lower Ludlow) or Middle Silurian 

 (Upper Wenlock) ; Hafod, Llandovery, Wales. 



Genus SCHUCHERTIA, Gregory. 



1858. Palasterina, Billings (pavt, not McCoy or Salter), Geol. Surv. Canada, Canad. Organic Kern., 



dec. iii, p. 76, pi. ix, fig. 1. 

 1862. Palasterina, Wright (part), Mon. Brit. Foss. Echinod., Oolitic, vol. ii (Mon. Pal. Soc, vol. 



for 1861), p. 26, fig. 16 b. 



1899. Schuchertia, Gregory, Geo). Mag. [4], vol. vi, p. 351. 



1900. Trentoriaster, Stiirtz, Verhandl. Naturh. Ver. preuss. Rheinl., Jahrg. 56, pp. 224, 225 (based 



on the same genoholotype as Schuchertia). 



1914. Schuchertia, Schuchert, Fossiliuin Catalogus, Animalia, pt. 3, pp. 5, 8, 30, 38, 43. 



1915. „ Schuchert, Bull. 88, U.S. Nat. Mus., pp. 51, 140, 152, 194, 195, 252. 



Generic Characters. — See p. 205. 



The genoholotype of Schuchertia is 8. stellatd, which was described more than 

 fifty years ago by Billings as Palasterina stellata. The species is found in the 

 Trenton (M. Ordovician) of Canada. Both Gregory and Stiirtz, about 1899, 

 recognised that the species was not a true Palasterina. Gregory gave the name 

 Schuchertia. Stiirtz, who published a little later, suggested the name Trentonaster. 



In 1915 Schuchert described two new species, S. lo.ro hi and 8. ordinaria, found 

 respectively in the Upper Ordovician and basal Silurian of the United States. I 

 am now able to show that S. lo.ro to is found in the Upper Ordovician of Scotland, 

 and that a fourth species, S. ivenloclri, n. sp., occurs in the Middle Silurian of 

 Scotland. This extends the range of the species both geographically and in horizon. 

 It seems very probable, judging by the way the species succeed each other in the 

 ascending order- of the strata, that they are descendants of one another. 



The oral surface of Schuchertia (Text-fig. 154, p. 216) may be derived from that 

 of a form like Eoactis— (1) by the further differentiation of the infero-marffinalia 



