152 BRITISH PALEOZOIC ASTEROZOA. 



adambulacralia increase in size distally until they reach a point a little distant from 

 the base of the arm, where the arm has its greatest breadth. In consequence, the 

 arms appear slightly petaloid. 



Each adambulacral has a very distinct crest, which starts from the extreme 

 margin of the arm and runs across the distal portion of the ossicle so as to end in 

 a sharp point in the middle of the groove. Pustules for the articulation of the 

 spines were present on the crest. It is not easy to determine the exact number, 

 but I believe that there were about eight. The broken bases of the spines may 

 sometimes be seen in the depression between the crests. 



Displaced ossicles show proximal and distal hollows for the interadambulacral 

 muscles, and the great thickness of the adambulacralia. A view of the interior of 

 the groove is presented by two other arms. The characteristic knee-shaped bend 

 (see p. 134) is well seen. 



Measurements. — R : r : : 36 mm. : 6 mm. Breadth of arm at base is 6'4 mm. 



Horizon and Locality. — The Upper Ordovician (Caradoc beds) of the Quakers' 

 burying ground, near Welshpool. 



2. Salteraster? imbricatus (Salter). Plate XI, fig. 5; Text-fig. 98. 



1866. Palteaster imbricatus, Salter, Mem. Geol. Surv. Gt. Brit,, vol. iii, pp. 289, 407, 480, pi. \xiii 



fig. 8. 



1914. Tetraster? imbricatus, Schuchert, Fossiliuni Catalogus, Aniraalia, pt. 3, pp. 28, 42. 



1915. Tetraster? imbricatus, Schuchert, Bull. 88, U.S. Nat. Mus., pp. 168, 169. 



Material. — The only materials are two poor casts of the holotype of the species 



Tkxt-fig. 98. — Diagram-plan of some of the ossicles on the apical surface of Salteraster imbricatus. 



R., radialia. x 4. 



in the Museum of Practical Geology, Jermyn Street (No. 28364). The natural mould 

 from which the casts were taken is stated by Salter to be " in the collection of 

 Mr. Prosser," but I have been unable to trace it. 



Specific Characters. — Body stellate, arms convex. Radialia small. 



Description. — Only portions of two arms and of the disc are preserved. The 

 form seems to have been distinctly stellate, the arms being much more pointed than 



