Anatoiny of the Temnopleuridte. Ill 



made with a view of comparing the simihiv structures of Tern- 

 nechinus and Trigonocidaris when an opportunity presented 

 itself. It was also thought to be advisable to investigate the 

 real value in a classiticatory sense of the crenulation of 

 primary tubercles and the presence or al)sence of grooves 

 between the pores of a pair — two matters which have been 

 made a great deal too much of in the classification of the 

 fossil forms of Echinoidea. 



The Sutures of the Plates. 



The method of the union of the coronal plates of the Temno- 

 pleurinffi was discovered six years since, and the remarkable 

 dowelling of the opposed edges of plates was described in 

 species of TemnopJeurus, Salmacis, and Amhlypneustes, and 

 subsequently in Pleurechinus^ together with the nature of the 

 sutural groovings and pits (Journ. Linn. Soc, Zool. vol. xvi. 

 pp. 343 and 447). 



I have now had the opportunity of examining better speci- 

 mens of Amhlypneustes ovum ; and there is no doubt that the 

 knob-and-socket structure upon the sides of plates is very 

 well developed. The former specimens noticed were not in 

 good condition as regards preservation, and it is a fact that if 

 the fracture of any Temnopleurid is crumbling and very 

 white in tint, changes have gone on which tend to destroy 

 the appearance of dowelling, which, moreover, is not very 

 visible in wet specimens. 



Mespilia has the same junction-structure, and I find that 

 Holop)neustes purpurescens ^ Liitk., has knobs and sockets upon 

 the opposed surfaces of the plates, with the exception of the 

 component plates of the geometrical plates of the ambulacra. 

 I do not find dowelling upon these plates in any genus, 

 although it is recognized between the compound plates. 



It would be expected that the thick tests of such a species 

 as Microcyphus zigzag^ Agass., would present some anomalous 

 arrangement of plate-junction ; but the knobs and sockets 

 are in considerable numbers near the outer parts of the edges 

 of the plates. On the plates on either side of the median 

 ambulacral suture (PI. XI. fig. 1) there are also some more 

 or less straight rows of knobs or sockets, passing from within 

 towards the outer part of the edge. This Microcyphus has a 

 remarkable amount of union of the plates free from dowelling, 

 and it appears that the ambulacro-interradial vertical sutures 

 are inseparably united. In all other genera the test frac- 

 tures very readily along this line, but in Microcyphus zigzag 

 the test will break in the poriferous zone rather than separate 

 at the suture. 



