148 CATALOGUE OF THE BLASTOIDEA. 



Let us now proceed to consider this classification in more detail, and to study the 

 mutual relations of the genera which constitute the six families defined above. 



Order REGULAEES, E. & C, 1886. 



Definition. Pedunculate Blastoids with a symmetrical base, in which the radials 

 and ambulacra are all equal and similar. 



Remarks. Little need be said about this group, which comprises all the types of 

 Blastoidea that were recognized by Roemer in 1851 as belonging to this class. He 

 was acquainted with Austin's Astrocrinus, but was inclined to exclude it from the 

 Blastoidea on account of its apparently tetramerous symmetry 1 ; while the allied 

 genus Elcutherocrinus had not then been discovered. 



Neither of these two genera, nor Pentephyllum, has any indication of the presence 

 of a stem ; while this is not known to have been wanting in any of the regular 

 Blastoids, in which also the five radials and the ambulacra corresponding to them are 

 perfectly homogeneous in character. All the interambulacral angles at the peristome 

 are of equal size, and no radial nor ambulacrum is different from its fellows. This 

 is so evident upon a glance at any of our first sixteen Plates, that nothing more need 

 be said upon the subject. 



The regular Blastoids fall very naturally into five families, three of which are typi- 

 fied by genera recognized by Roemer, viz. Pentremites, Elceacrinus, and Codaster. The 

 leading forms of the other two families, Granatoblastidce and Troostoblastidce, are the 

 species which he regarded as the types of his Pentremites EUiptiei and Pentremites 

 Clavaii respectively. It will be evident, therefore, that our classification has followed 

 very closely upon the lines which he had laid down when the science of Blastoid 

 morphology was as yet in its infancy. We believe, however, that there are some new 

 features in it ; but we are quite prepared to hear Mr. Hambach's valuable criticism 2 

 again repeated: — "The mere recapitulation of what has been clone by others is of 

 very little value, even if a different terminology is used for it." 



Family PENTREMITID^E, d'Orbigny, 1852 (emend., E. & C, 1886). 



Definition. Base usually convex and often much elongated. Spiracles five, but 

 sometimes more or less completely divided by a median septum. Their distal 

 boundary formed by side plates. Hydrospires concentrated at the lowest part of the 

 radial sinus. 



1 Archiv f. Naturgesch. 1851, Jahrg. xvii. Bd. i. p. 390. 



2 Trans. St. Louis Acad. Sci. 1S84, vol. iv. no. 3, p. 547. 



