286 ROBERT TRACY JACKSON ON ECHINI. 



respect they are somewhat lower than in A', cotteaui as described, but much higher than in 

 species of Lepidocentrus. A general depressed area surrounds the pore-pair, extending on to the 

 next plate in this internal view. This leaves an elevated ridge between succeeding pore-pairs, 

 and the general superficial appearance resembles the ribs of a turtle in relation to the carapace. 

 ^Imbulacral plates imbricate adorally and bevel strongly under the adambulacrals. (As this 

 is an internal view, the exact opposite appears in the specimen; that is, ambulacral plates in 

 this view imbricate aborally and laterally bevel over the adradials; compare text-fig. 37, p. 75.) 



The interambulacrum is composed of eight columns of plates, which are nearly rhombic 

 in outline, much as in Lepidocentrus mulleri. A median column is narrower than the others 

 and in both areas this is column 7. The plates imbricate strongly aborallj* and from the center 

 laterally and over the ambulacrals. (As this is an internal view, the exact opposite appears 

 to be the case; that is, interambulacral plates in this view imbricate adorally and under the 

 ambulacrals; compare text-fig. 34, p. 75.) The ventral part of the test is unknown, but 

 dorsally an ocular which is low but exceptionally wide, covers ambulacrum B and in \)SiYi 

 interambulacra A and C on either side. A very low genital is in A and a part of one in C. 

 A small plate between genital A and ocular B may be a displaced plate, but it appears as if 

 the periproct met the interambulacrum in this area like the condition common in Recent Echino- 

 thuriidae (text-fig. 170, p. 149; p. 287). The apical disc in this species is evidently small in pro- 

 portion to the diameter of the test, a progressive character. A few small angular plates lying 

 dorsal to the ocular and genitals are apparently remnants' of the periproct. As the exterior 

 of the plates is unknown, surface characters cannot be given; but thej^ are probably as in 

 K. cotteaui. 



This choice and beautifully preserved specimen was collected in 1909 by Mr. A. W. Giles 

 from the base of the Rochester Shale, about ten feet above the Irondequot Limestone, in the 

 Niagara Group, Silurian. It was found in these beds in a ravine of the Genesee River in the 

 north part of Rochester, New York. On the slab are fragments of other fossils including a 

 faceted eye of Dalmanites limulurus Green, a typical Niagara Group trilobite. The holotype 

 with two additional fragmentary pieces is in the collections of the University of Rochester, 

 at Rochester, New York, and was loaned to me by my friend. Professor H. L. Fairchild of that 

 institution. 



Lepidocentrus Miiller. 



Lepidocentrus Muller, 1857, p. 258; Schiiltze, 1866, p. 123; Loven, 1874, p. 39; A. Agassiz, 1881, pp. 79, 

 80; Duncan, 1889a, p. 9; .lackson, 1896, p. 222; Lambert and Thiery, 1910, p. 121. 



Test spheroidal, ambulacral areas narrow throughout, plates low, imbricating adorally 

 and beveled under adambulacrals; pore-pairs uniserial, situated toward the next adjacent 

 interambulacrum, as seen externally, but near the middle of the plate as seen internally. Inter- 



