REPORT ON THE ECHINOIDEA. 17 



that of the Echinidse. Among the new genera of the Diadematidae, the spines of 

 Micmpyga are found to be closely allied in structure to those of Astropyga, but with a 

 comparatively larger central foramen and a greater number of calcareous wedges 

 (PL XXXIX. fig. 1) ; while a section across the swollen tip of a spine of the actinal sur- 

 face (PI. XXXIX. fig. 2) shows that the solid wedges are reduced to a mere sheath with 

 an inner ring of closely-packed cells arranged in radiating lines, with a dense reticulated 

 central pith. 



In Asindodiadema we find indicated, by the structure of the spines as well as by the 

 structure of the test, the afiinities of the genus with the Cidaridse. We find the outer 

 row of calcareous wedges characteristic of the Diadematidse (PI. XXXVIII. figs. 17-1 9), 

 with the coarse reticulation of the contents of the shaft as in Porocidaris and Salenia, 

 and the pith of the central part formed of large and coarse reticulations. 



Mackintosh ^ has already given the structural features of the spines of Asthenosoma. 

 The structure of the spines of Phormosoma (PI. XXXVIII. fig. 4 ; PI. XXXIX. figs. 3-9) 

 agrees in the main with that of Asthenosoma. These show close affinity on the one side 

 to the Diadematidaj (PL XXXVIII. figs. 1, 2, 4, 5, 6 ; PL XXXIX. fig. 6), and on the 

 other to the Arbaciadse, in the large irregular I'eticulation characteristic of the interior 

 of the shaft (PL XXXIX. figs. 3-7). 



Mackintosh has also figured the same open reticulation in the central part of the 

 shaft of the spine of Ecliinothrix, and has called attention in that genus to the vari- 

 able character of the solid calcareous wedges forming the outer sheath of the spines 

 (compare PL XXXIX. figs. 3, 4, and 6). Among the Echinidse, even this structure of the 

 interior of the shaft is still found in Pseudoholetia (PL XXXIX. fig. 11), and it also exists 

 among the Clypeastroids in spines either with or without a central cavity (PL XXXVIII. 

 figs. 21, 22 ; PL XXXIX. fig. 19). In fact the spines of the genera of the Clypeastridae, 

 which I have thus far examined, Encope, Mellita, and Clypeaster (PL XXXVIII. figs. 

 19-22), are interesting from the decided afiinities they show to the Echinidse in the 

 small number of the solid wedges of which they are composed, and in their great 

 resemblance to the monocyclic types of structure of the Triplechinida3, only that in the 

 Clypeastridge the shafts are foraminated. In the Petalosticha a similar large foramen exists 

 in the majority of the genera (see Pis. XXXVIIL, XXXIX.), and with the exception of the 

 genera Hemiaster, Argopatagus, and Rhynchopygus, in which the wedges resemble more 

 those of the Clypeastroids, we find that the outer ring of large wedges is generally made 

 up of triangular pieces with a rounded apex towards the inner foramen ; these wedges are 

 few in number, in some of tlie genera not more than nine (PL XXXIX. figs. 12, 28, 30, 

 34, 36). This appears to be the general structure of the spines of the Pourtalesise, and 

 the allied genera, while in the Spatangoids proper, such as Lovenia, Breynia, Eupatagus, 

 and the Ananchytid-like Homolampas and Linopneuste-s, the outer ring is made up of a 



' Trans. Roy. Irish Acad., xxv., 1875. 

 (ZOOL. CHAU.. EXP. PART IX. — 1881.) I 3 



