21 '2 BRITISH PALEOZOIC ASTEROZOA. 



very narrow ambulacral grooves. These ossicles are subquadrangular in outline 

 near the base of the column, but become wider than long distally ; there are about 

 15 in a column, terminating proximally in two larger, prominent, wedge-shaped 

 plates of the oral armature. Interradially upon each pair of oral pieces is placed 

 a large, single, pentagonal plate (holds the position of axillary interbrachials), 

 against which rest two diverging inframarginal columns, each with 13 or 14 

 ossicles, and these columns continue adjoining the adambulacrals. Before attaining 

 the distal ends of the rays they gradually become smaller and pass over to the 

 abactinal side. Other actinal disc plates are also present, but apparently are 

 arranged in quincunx, and are smooth like those of the abactinal side. 



" Ambulacrals unknown." 



Measurements. — The holotype gives R : r : : 6 mm. : 3 mm. The second speci- 

 men gives R : r : : 9 mm. : 4 mm. 



Horizon and Locality. — Middle Ordovician (Trenton Limestone) ; Canada. 



2. Schuchertia laxata, Schuchert. Plate XV, fig. 1 ; Text-figs. 150—153. 



1914. Schuchertia laxata, Schuchert, Fossilium Catalogus, Animalia, pt. 3, p. 38. 



1915. „ „ Schuchert, Bull. 88, U.S. Nat, Mus., pp. 198—9, pi. xxxii, fig. 3 ; pi. xxxiii, 



figs. 2, 3. 



Material— Four moulds in Mrs. Gray's Collection from the Starfish Bed of 

 Thraive Glen (nos. D. 283, D. 330— D. 332), all fragmentary. D. 283 shows a 

 portion of the apical surface of an arm. The apical ossicles have fallen away, 

 exposing the ambulacralia (Text-fig. 120, p. 180). D. 330 shows the apical surface 



Text-fig. 150.— Apical plates of Schuchertia laxata (after Schuchert). x 8. 



of a portion of two arms and is figured PI. XV, fig. 1, and Text-fig. 151. Views 

 of portions of the oral surface are given by D. 331 and D. 332. The plates are 

 much displaced in the former specimen. The latter shows only portions of two 

 arms but they are in good condition. 



Schuchert notes that in America " six specimens are known, four of which are 

 poorly preserved and but a jumble of plates. The specific name is given to 

 indicate the generally separated condition of the plates." The British material 

 repeats this condition. 



