GENTIS PLETJEECHINTJS, L. AGASSIZ. 453 



Agassiz pits, are deep, and are increased by the elevated ridge- 

 like ornamentation which carries the tubercles ; they are marked 

 on their floor by the margin of the suture, or where two conti- 

 guous plates join ; and they dip inwards at the sutural angle, 

 penetrating and expanding slightly, and leaving but a thin layer 

 of reticulate test between them and the interior of the test. On 

 the edge of the plate, on either side of the true pits, are processes 

 with knobs and sockets, and these are larger and fewer in 

 number than in Temnopleurus toreumatictos-. Between the inter- 

 radial coronal plates and the tentaculiferous plates there is the 

 same kind of suturing as in Temno])leurus. Knobs on the ambu- 

 lacral edges, and sockets on the interradials, with considerable 

 undermining of the true pits here and there. The knobs are 

 larger, however, than in Temnopleurus. The horizontal sutures, 

 actinal and abactinal, show the knob-and-socket arrangement 

 very feebly ; the knobs and sockets are few in number, and are 

 not seen all along the adjoining plate-edges beneath the depres- 

 sions, but only on the sutures where the raised ornamental ridges 

 join. There a few sockets fit corresponding knobs on the other 

 plate ; the distribution, however, of the knobs and sockets is as 

 it is in Temnopleurus, and there is the same difi"erence in the rela- 

 tive position of the knobs and sockets in the interradial and ambu- 

 lacral areas. 



Einally, not only pedicellarise with stout heads on long stalks 

 are close to the depressions over the horizontal sutures, but there 

 are rather long-headed pedicellarise on short stalks immediately 

 around the edge. There is often a definite indication that not 

 ouly are these depressions and true pits lined with membrane 

 which does not carry pedicellarise or any structures, but that a 

 layer of tissue covers in, more or less, the depressions, not at the 

 level of their edge, but a little lower. It appears to be incom- 

 plete, and to have a slit-like opening in it, so that the deeper 

 part of the depression and the true pit are more or less covered in. 

 Neither ova nor young are found in these depressions, although 

 their marsupial character is very present to the imagination. 



In minute construction, there is a generic relation between 

 Temnopleurus and Pleurechinus, and the only important distinc- 

 tion is the absence of crenulation in the last-named type. The 

 value of this distinction is not great ; but when certain series 

 of forms have crenulate tubercles without exception, and one 

 occurs, closely allied by its minute structures, having non- 



