EEPORT ON THE ECHINOIDEA. 81 



zone. The teeth are grooved as in the CidariclEe. The face of the pyramid of the jaw is 

 like that of the recent Echinothuridse, remarkable for the length of the teeth compared to 

 the height of the pyramid which is much less than is the case in the Echinidse. The 

 upper foramen of the pyramid is very small, the cheeks of the pyramids are deeply cut 

 by a triangular pit which forms a narrow edge, the outer side of the p}Tamid, and a well- 

 marked divitling wall with parallel edges between these two deep triangular depressions, 

 a modification of the structure of the pyramid, which as yet has not been seen in any of 

 the recent Echinidse. See also Mtiller's figure of the p)Tamid (Neue Echinod. d. Eifeler 

 Kalk., pi. iii. fig. 12). From the drawings of Trautschold of Archceocidaris rossica the 

 jaws of the genus resemble most closely those of Cidaris. 



In the Cystocidaridas of Zittel {Echinocystites of Wy. Thomson and HaU) the jaws are 

 apparently very difi'erent from those of the Perischoechinida3, but they are not sufficiently 

 well known to compare them to those of other Echinoderms, though they would appear 

 from the drawings of Thomson to approach somewhat the mouthpieces of the Starfishes 

 and Ophiurans. The anal system of this group of Echinids is also excentric and not placed 

 at the apex of the test, where the other plates of the apical system are placed, — a 

 structural feature which was supposed to be characteristic only of the higher Ecliiuoidea, 

 the Spatan golds and Clypeastroids. 



But by far the most embryonic of all Echinoidea, and the most interesting of the 

 Palaeozoic Sea-urchins, is the remarkable genus Bothriocidaris, in which, if the figures of 

 Schmidt ^ are correct, there is absolutely nothing to distinguish the plates of the actinal or 

 abactinal systems from those of the coronal plates proper in the ambulacral and interam- 

 bulacral areas. The ambulacra extend unbroken to the very plates which constitute the 

 edge of the actinal opening, and the interambulacral areas, which consist only of a single row 

 of plates, to the last row of plates surrounding the actinostome; and at the abactinal pole 

 the plates pass similarly without any change into the minute plates which must have 

 made up the anal system. As in all young Echinids the test also is made up of a small 

 number of coronal plates very uniform in size on both the ambulacral and interambulacral 

 zones, and diminishing only slightly in size towards the actinal and abactinal systems. 



It seems quite evident from the above, and from the examination of the species of 

 Phormosoma and Asthenosoma collected by the Challenger, that the Palseechinidse are far 

 more closely allied to the recent Echinids than is usually supposed, and that we have in the 

 recent Echinothuridse structural features combining the characteristics of the normal 

 Desmosticha and of the Palajechinidse to such an extent that we are hardly justified in 

 regarding the Palaeechinidas as a subdivision of the Echinoidea equivalent to that of the 

 Cl3q5eastroids or Spatangoids. 



In the Echinothuridse the pedicellarise in their general character are allied to those of 

 the Diadematidse, more specially to those of Astropyga. The long-headed long-stemmed^ 



1 Fr. Schmidt, Neue Bait. Sil. Petref., Mem. Acad. St Petersb., vii^. S^rie, xxi., pi. iii. 



ZOOL. CHALL. EXP. PART IX. — 1881.) I H 



