298 CATALOGUE OF THE BLASTOIDEA. 



elongately triangular, extending upwards to support the azygos radial. Four of the 

 radial plates are generally alike, A and B being slightly different from C and E, which 

 have modified posterior limbs ; but the relative proportions of the bodies and limbs, 

 and the length of the latter vary greatly. The fifth radial is shorter than the others, 

 somewhat rhombic in outline, not forked or divided into body and limbs, but simply 

 emarginate above, resting on the sloping upper edges of the two similar basals. 

 Deltoid plates dissimilar, three of them roughly oblong and subequal in size, the two 

 others much smaller, and placed at the sides of the odd ambulacrum. Four of the 

 ambulacra are lanceolate-linear, with side and outer side plates as usual, and almost 

 reach the dorsal surface ; lancet-plates partially exposed throughout the whole length 

 of the ambulacra; the fifth or right postero-lateral ambulacrum (D) is modified, being 

 shortened, subtriangular, and horizontal. One hydrospi re-fold on each side of the 

 radial sinus. Spiracles unknown. Mouth large. Anus unknown. Ornament con- 

 sisting of tubercles and spines. 



History. This interesting genus was originally proposed by the Messrs. Austin in 

 1842, and briefly defined a year later. Dr. Bronn, in his wonderfully accurate 

 ' Nomenclator,' rejected the name Astrocrinites on account of its resemblance to 

 Asterocrinites, Minister, and proposed Zi/f/ocrinus in its place. Our friend Prof. F. 

 Roemer published a few remarks on this genus in 1851. On account of its appa- 

 rently tetraradiate structure, he considered it to have a greater resemblance to the 

 Cystidea than to the Blastoidea. On the other hand, Messrs. de Koninck and Le Hon 

 referred it to the latter group. In 1854 Prof. John Morris altered Austin's name 

 to the form by which it is now generally known, Astrocrinus. The late Prof. Pictet, 

 in 1857, followed Bronn, and de Koninck and Le Hon, by using the term Zygocrinus, 

 and, like the latter authors, he placed the genus among the Blastoidea. From this 

 time forward nothing more appears to have been written on Astrocrinus until 1876, 

 when one of the present writers described a second species from the Scotch Carbo- 

 niferous Limestone Series, and entered into the general question of the affinities of 

 the genus. Although some of the statements contained in this paper have required 

 subsequent modification, more especially those relating to the odd ambulacrum, the 

 general description of the type and the views therein set forth still hold good. 

 Lastly, in 1883, the present writers conjointly effected the required change in the 

 nomenclature of the various parts of Astrocrinus, and pointed out its affinities with 

 Eleutkerocrinus. 



Remarks. This aberrant member of the Blastoidea is distinguished from all other 

 genera of the class, with the exception of Eleutkerocrinus (PI. XIX. fig. 6), by the 

 peculiar modification, or apparently abortive condition, of one of its ambulacra. 

 This gives a tetraradiate appearance to the calyx, although, so far as the ambulacra 

 are concerned, its symmetry is pentamerous, as in all Blastoids. It is further peculiar 

 but possibly not unique in the presence of superficial tubercles which had minute 



