400 ROBERT TRACY JACKSON ON ECHINI. 



over the ambulacrals. The adambulacral plates are pentagonal and rounded on the ad- 

 radial sutures. Plates of the median columns are hexagonal, but the angles of all the.se plates 

 are somewhat rounded, not sharply defined, as in the Palaeechinidae. There are secondary 

 tubercles only on ambulacral and interambulacral plates. No spines are preserved in the 

 type, but in a second specimen in the Museum of Comparative Zoology 3,186, there are fine 

 tapering spines, swollen at the base, and measuring up to about 2.5 mm. in length. Ven- 

 trally, the primordial interambulacral plates are in the basicoronal row, and, passing dorsallj^, 

 new columns are progressively added until the full complement is attained below the mid-zone. 



Although only partially preserved, it may be safely stated that there are only ambulacral 

 plates on the peristome. The dorsal portion of the test is unknown, and the lantern is not 

 preserved in the type, but in another choice specimen in the Museum of Comparative Zoology 

 no. 3,186, from the Wachsmuth Collection, there is a well preserved lantern in place. The 

 pyramids are wide-angled, with ridges for the attachment of interpyramidal muscles, foramen 

 magnum moderately deep, and teeth grooved, all as usual in Palaeozoic species. Some ambul- 

 acral plates in this specimen are evidently peristomal plates. 



This species, the type of the genus, differs from the other species in that it has more columns 

 of interambulacral plates, also more strongly imbricate plates, both in the ambulacra and the 

 interambulacra. The specimen which I identify as the type has no original label, but bears 

 the diamond-shaped green ticket that Professor Hall used to indicate types. It agrees entirely 

 with Hall's original description. Moreover, the late Professor Whitfield informed me that the 

 type was in the Barris Collection, which was purchased by Professor Louis Agassiz for the 

 Museum of Comparative Zoology. Professor Hall in his description says that the interam- 

 bulacra] plates imbricate adorally, "the lower edges of each range overlapping those below; 

 while the plates of the ambulacral areas are imbricating in the opposite direction." The 

 fact that he said that interambulacral plates imbricate adorally is ascribed to incorrect orienta- 

 tion, which has led to a similar interpretation in regard to imbrication in some other cases. 

 Indeed, Professor Hall (1868, p. 296) himself said that he had probably given the direction of 

 imbrication incorrectly in his original description. Professor Hall pointed out that the essential 

 difference of the genus (and species) from Palaeechinus is the fact that the plates are imbri- 

 cate. He also considered as a difference the number of columns of interambulacral plates, but 

 this with present understanding is not considered a generic difference. The species differs 

 generically from what Hall described as Lepidechinus rarispinus, which species is here referred 

 to Hyattechinus (p. 292) and figured on Plate 21, fig. 6 and Plates 22, 23. 



Burlington Limestone, Lower Carboniferous, Burlington, Iowa, holotype. Museum of 

 Comparative Zoology Collection 3,055; another choice specimen especially good for the ventral 

 part of the test, lantern, and spines. Museum of Comparative Zoology Collection 3,186; also a 

 fragmentary specimen in the same museum no. 3,056 (original no. 417). 



