, 



236 Messrs. R. Etheridge, Jun., and P. H. Carpenter on 



when mature ; though the apparent absence of any stem-facet 

 in the fossils may be merely the result of weathering, and not 

 due to natural obliteration during life, as in the Comatulce just 



mentioned. 



According to the Messrs, Austin there is in A. tetragonus 

 an oval eminence near the centre of the azygos basal, " appa- 

 rently analogous to the madreporiform tubercle " of the 

 Echinozoa. There is certainly nothing like a madreporite in 

 the Scotch species A. Benniei ; and we imagine the struc- 

 ture in question to be nothing but the fold which has been 

 already described as marking the union of the azygos basal 

 with the two longer ones. 



In the former description of A. Benniei by one of us, refe- 

 rence was made to a small spine, which, although not exhi- 

 biting any traces of definite attachment to the specimen to 

 which it adhered, was believed to belong to A. Benniei. A 

 closer examination of the ornamenting tubercles has led us to 

 the conclusion that some of the larger ones, at any rate, were 

 perforate. This fact has also been independently observed by 

 our friend, Mr. P. Highley, whilst engaged in figuring the 

 specimens ; and in our forthcoming monograph illustrations 

 will be given exhibiting this feature. It is not unnatural to 

 suppose, therefore, that some probability exists of the spine 

 previously referred to belonging to A. Benniei. 



While appearing to agree with Eleutherocrinus in general 

 structure, so far as this can be made out, Astrocrinus presents 

 several points of difference from that type. It is much smaller 

 and altogether dissimilar in appearance, being flattened and 

 more or less distinctly stellate or lobate. The outline varies 

 considerably, the anterior lobe (t. e. that opposite the azygos 

 ambulacrum) being sometimes considerably produced and 

 sometimes comparatively short. The four normal ambulacra 

 cross one another nearly at right angles. This is very far 

 from being the case in Eleutherocrinus, where they only occupy 

 180° of the summit, as is well shown in Shumard's figure ; so 

 that the odd ambulacrum takes up a relatively larger portion 

 of the summit than in Astrocrinus. The latter genus is 

 limited to the British Carboniferous Limestone, while the two 

 American species of Eleutherocrinus are both from the Hamil- 

 ton group (Upper Devonian), The two genera are so entirely 

 different from all other Blastoids that they must be placed in 

 a family by themselves ; and in naming this it is only right 

 to use the term Astrocrinidse, T. & T. Austin, 1843. Another 

 genus (AporocriniteSj Austin, MS.) was also included in 

 this family by its founders ; but it has never been described, 





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