a neio Genus of Echinodermata. 413 



quence of their being elements of structure supposed to be en- 

 tirely unrepresented in Asteroids, is placed in quite a different 

 light by the statement made by A. Agassiz that homologous 

 plates occur in an early stage of the Asteroid larva, but ulti- 

 mately become resorbed during the growth of the ambulacra. 



5. The prolongation of the peritoneal cavity into the radial 

 portion of the animal is a divergence towards a structure 

 usual in the Asteroid group*. 



6. The simple and rudimentary character of the mouth- 

 armature certainly follows much more closely in principle the 

 plan of structure obtaining in Asteroidea than the highly 

 modified and complex organization which is found in most 

 Ophiuroids. In fact simple mouth-plates bearing papilla? 

 only are in Astrophiura the sole representatives of the ela- 

 borate apparatus, consisting of side mouth-shields, jaws, jaw- 

 plates, and teeth, which is met with in the latter group. 



It may be suggested that the mouth-plates, which are 

 spoken of in the foregoing description as representatives of 

 the side mouth-shields, might with more propriety be styled 

 "jaws," since these latter are the first formed and are the 

 result of the development of the first adambulacral plate, 

 whilst the side mouth-shields are the modification of the 

 second adambulacral plate. A moment's consideration, how- 

 ever, will show that any dogmatic insisting upon such a 

 nomenclature is a mere play on words ; for it will be seen that 

 the term which I elected to use, for descriptive purposes only, 

 was that which expressed most clearly the position of the 

 plates in relation to the mouth-shield, as well as their own 

 individual form ; whilst the fact of their wanting both jaw- 

 plates and teeth would have rendered the application of the 

 other term, if perhaps somewhat more precise, certainly much 

 more misleading. 



7. The aborted character of the axis or internal arm-ske- 

 leton is particularly noteworthy, and gives evidence of a 

 divergence from a very characteristic Ophiuroid structure, 

 from which due significance must not be withheld when 

 formulating the affinities of the present animal. 



In conclusion, it may be noted here that Ophiophyllum 

 petilum, Lyman f, a strangely abnormal Ophiuroid taken 

 during the ' Challenger ' expedition, superficially resembles 

 Astrophiura in having a fringe-like border of spines round 

 the margin of the disk : but here the comparison ceases ; for 

 they are not homologous with the same appendages in our 



* So far as the ovaries are concerned, Mr. Lyman informs me that he 

 thinks he has found some such extension in Ophiocreas. 



t u Ophiuridre .and Astrophvtidre of ' Challenger ' Exped.," Bull. Mus. 

 Comp. Zool. vol. v. p. 180, pi. vii. figs. 170-181. 



