SYNTHETIC TYPE OF OPHITJKID. 77 



tooth. The regularity of the pentagon surrounding the oral 

 apparatus is very striking, and so is the extreme separation of the 

 jaw-angles, much of which, however, may be due to post mortem 

 contraction. All the plates on the upper surface of the disk have 

 separate, broad-based, two- or three-thorned, short spinules on 

 their edges and rarely elsewhere, but the spinulation is not dis- 

 tinct between them. The radial shields have the greatest number 

 of spinules on them. All the spines on the side arm-plates pro- 

 ject at right angles to the arm, and the hooks are glassy at their 

 top. The combination of Amphiuran characters and those of 

 Ophiothrix is thus remarkable. 



Miiller and Troschel established the genus Ophionyx and gave 

 its diagnosis in their ' System der Asteriden,' 1842. It has the 

 disk furnished with isolated many-thorned spinules, the mouth 

 has only tooth-papilla?, there are two generative openings in 

 each interbrachial space, and the arm is furnished beneath w r ith 

 echinulate spines and hooks. Ophionyx armata, M. & T., is de* 

 lineated by them and O. scutellum, Grrube, is noticed. This genus 

 can hardly be separated from Ophiothrix ; and although Ophionyx 

 armata is not without the aspects of the form now under con- 

 sideration, the structural distinctions of the absence of tooth- 

 papilla? and the presence of accessory plates to the upper arm- 

 plates are incompatible with the union of the species under one 

 genus. 



The genus Ophiopholis, Mull. & Trosch., has the upper arm* 

 plates surrounded by a rim of minute accessory plates, and the 

 lower spine of the under arm-plates is a hook ; moreover, the disk 

 is more or less covered with grains or little spines*. There 

 are mouth-papilla? on the sides of the jaw-angles. In Ophio- 

 lepis, Mull. & Trosch., the disk has naked plates or scales, there 

 are small accessory scales on the disk and arms, a row surround- 

 ing the disk-plates ; there are mouth-papilla?, and the arm-spines 

 are arranged along the outer edge of the side arm-plates, and there 

 are usually two tentacle-scales. It is evident, as was suggested at 

 the commencement of this communication, that the alliances of the 

 form are more with these last two genera, but still the distinctness 

 is decided. The extreme simplicity of the dental apparatus, there 

 being no tooth- or mouth-papilla? on the jaw-angles, only a spine 

 on the side mouth-shield or arising from its junction with the 

 jaw, and evidently a tentacle-scale, is remarkable j the true teeth 

 are well developed. The disk is symmetrically plated, spinules 

 * See Liitken, Addit. ad hist, Ophiiirid. p. 11, pi. ii. fig. 16a (1861). 



