On Palceodiscus and Agelacrinus. 31 



" On the Structure and Affinities of Palseodiscus and Agelacrinus." 



kBy W. K. SPENCER, B.A., F.G.S., Burdett Coutts Scholar, 

 University of Oxford. Communicated by Professor W. J. 

 SOLLAS, F.RS. Received March 22, Read June 9, 1904. 

 [PLATE 1.] 



The following research was undertaken at the suggestion of 

 Professor Sollas. It endeavours to elucidate, by the aid of the method 

 described in the 'Phil. Trans., 5 B., vol. 196, the structure of two 

 genera of rare but very important Palaeozoic Echinodermata, namely, 

 the Echinoid Palseodiscus and the Edrioasteroid Agelacrinus. The 

 iossils were ground at uniform distances of T ^ mm., and each successive 

 surface photographed. From tracings of the photographs wax models 

 were constructed. On the results of these investigations views are 

 brought forward as to the relationships of some of the groups of the 

 Echinodermata. I have to thank Professor Sollas for his never-failing 

 advice and assistance, and also Dr. Bather for his courtesy in my visits 

 to the British Museum of Natural History. 



Palceodiscus ferox (Salter). 



Palseodiscus was discovered by Salter (1) in the Lower Ludlow beds 

 at Leintwardine, Shropshire. The specimen was in the form of a cast, 

 and otherwise imperfect. Salter was unable to decide whether " it was 

 an Ophiurid with contracted arm plates or one of the Asteriadse with a 

 greatly developed masticatory apparatus." 



Wyville Thomson (22) in 1861 described specimens contained in the 

 College Museum in Belfast. He placed Palseodiscus in near relation- 

 ship with Echinocystites, a genus which he here describes for the first 

 time. Both genera were recognised as Echinoids, but with close 

 relationships to the Cystoids. Wright in his monograph of the ' British 

 Fossil Echinodermata of the Oolitic Formations ' was apparently unaware 

 of Wyville Thomson's' work, and merely copies Salter's descriptions. 

 Zittel, in 1879, while recognising the Echinoid affinities of Echino- 

 cystites, yet places Palseodiscus amongst the Asteroidea. Neumayr, in 

 1881, regarded Palgeodiscus and Echinocystites as very nearly related. 

 " Im Ganzen stellt Palseodiscus einen Typus vor, welcher Charaktere 

 von Seesternen mit solchen von Cystocidaris (Echinocystites), der 

 wichtigen Zwischenform zvrischen Seeigeln und Cystideen, verbindet." 

 (16, p. 419.) 



Gregory (5, p. 130), in 1897, definitely placed Palseodiscus amongst 

 the Echinoids. He remarks, " the general characters of the skeleton are 

 more echinoid than asteroid; the absence of special adambulacral 

 plates, the occurrence of small articulating spines, the resemblance of 



