Vol. 55.] SILURIAN ECHINOIDEA AND OPHlTmOIDEA. 699 



The buccal armature^ occupies the greater part of the interior 

 of the test. It consists of five strong pieces, interradially placed, and 

 meeting in the centre to form the dental rosette. Each piece appears 

 to be traversed by a median longitudinal fissure, and is probably 

 composed of a pair of ossicles. 



The genus Eutliemon differs from Eudadia in the absence of 

 imbrication of the plates that form the test. 



On a review of the characters presented by the three species just 

 described, it will be admitted that their affinities are with the 

 Ophiuroidea. The absence of any openings on the dorsal surface, and 

 of any indication of an anus, the ventral position of the raadreporite, 

 and the sharp distinction of the arms from the test indicate this 

 much. On the other hand, they differ from all known Ophiuroids 

 in several important particulars. 



The structure and disposition of the arms is unlike anything 

 known among either the Ophiurae or the Euryalae, and finds no 

 parallel among any group of fossil Ophiuroidea. If we consider the 

 disposition of the arms first, we find as a constant character among 

 the rest of the Ophiuroidea the extension of five of these appendages 

 over the ventral surface of the disc as far as the buccal aperture : 

 the vertebral ossicles of the arms are also serially represented in 

 the buccal armature. In the Eucladidae also the arms are given off' 

 from the ventral surface of the test, and the first pair have their 

 origin in the outer distal angle of the jaws : if, however, we are to 

 regard the serial arms of Eucladidae as the free extremities of lateral 

 branches given off in pairs from a median hypothetical arm, then 

 we must admit that the branching takes place within the test to an 

 extent otherwise unknown within the class. ^ If we turn next to 

 the structure of the arms, we find no less striking peculiarities. No 

 decisive evidence exists to prove that vertebral ossicles are present, 

 but if they are they must be out of all proportion small compared with 

 the lumen of the arm. Since the cavity of the arm is almost entirely 

 unoccupied by skeletal structures, the question naturally arises as to 

 the nature of the soft parts which it contained. The distal arms are 

 so large that they might well have afforded room for extensions of 

 the digestive viscera from the test. The absence of visible apertures 

 in the arms is another very puzzling feature, and one is almost tempted 

 to enquire whether the paired appendages are to be compared 

 with arms at all. 



The nature of the buccal armature is very different from that 



^ The buccal armature is seen in the middle of fig, 4, separated by an annular 

 space from the wall of the test. This space was probably covered in the living 

 animal by a buccal membrane. 



^ In connexion with this, Mr. Minchin has called my attention to the 

 remarkable Ophiuroid described by my friend Mr. W. P. Sladen under the name 

 of AstropMura permira, Ann. & Mag. Nat. Hist. ser. 5, vol. iv (1879) p. 401. It 

 presents several striking analogies with the Ophiocistia. The arms are not 

 paired, it is true, but single and much reduced ; they are, however, incorporated 

 with the test to an unusual extent. The vertebral ossicles are peculiar and 

 rudimentary, and the buccal armature is more Asteroid than Ophiuroid in 

 character. 



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