294 ROBERT TRACY JACKSON ON ECHINI. 



On account of the exceptional quality and interest of the material, some detailed discussion 

 of the specimens is desirable. The type (Plate 22, fig. 7; Plate 21, fig. 6) is a fine sandstone 

 mold of the dorsal portion of the test (p. 395). It has three ambulacra and interambulacra 

 preserved, and its periphery coincides with the mid-zone. This was the only specimen of this 

 species known to Hall, as his second specimen is considered here as belonging to a distinct species 

 and genus, namely Lepidocentrus whitfieldi, p. 290. In this type specimen the ambulacra are 

 narrow and the plates are relatively narrow and low, as it is a dorsal view. In each interambula- 

 crum there are eleven columns of plates, the eleventh originating above the mid-zone as shown. 

 In each interambulacral area the adambulacral columns drop out in passing dorsally, as a senes- 

 cent character. As it is an external mold, the lettering is reversed from what it would be if 

 seen from the interior (p. 21). 



The Yale Museum specimen shown in Plate 22, fig. 8, Plate 23, fig. 3, is also a sandstone 

 impression of the dorsal side, but from Warren, Pennsylvania. Being an external mold, there- 

 fore the lettering is reversed, and it is the most nearly complete dorsal area known. The ambu- 

 lacra are narrow and plates relatively narrow and low. Interambulacra A and G have eleven 

 columns of plates each. In area I there are twelve, and in C and E, thirteen columns of plates, 

 thus showing considerable radial variation. In each area the adambulacral columns 1, 2 drop 

 out dorsally, when columns 3, 4 assume an adradial position near the apical disc. Five radial 

 depressed pits appear, seen best in Plate 22, fig. 8, which are evidently due to a forcing up of 

 the test at these points when, in flattening, the lantern came in contact with the dorsal area. 



A remarkable specimen in the Yale Museum from Warren, Pennsylvania (Plate 22, figs. 1, 

 2; Plate 23, figs. 1, 2, 7) is one of the most interesting Echini that I have studied. As seen, 

 the specimen consists of counterparts which structurally supplement each other. In one of 

 these (Plate 22, fig. 1; Plate 23, fig. 1) we have for the most part an external sandstone mold 

 of the ventral side, showing beautifully the impressions of ambulacral, interambulacral, and peri- 

 stomal plates, with sutures, tubercles, and peripodia. With this there is in areas B, C, and 

 imperfectly in D, an internal impression of the dorsal side, molded on the sand which filled the 

 interior of the test. In the reverse counterpart (Plate 22, fig. 2; Plate 23, figs. 2, 7) we have 

 in part in areas J, A, B, and C an external mold of the dorsal side showing tubercles, etc., but for 

 the most part the specimen shows an internal impression of the ventral side molded upon the 

 sand which filled the interior of the test. On this part there are no tubercles or peripodia, 

 as it represents the proximal faces of the plates, but there are impressions of suture lines, pore- 

 pairs, the spinose projections from the interior of ambulacral plates, and the lantern. Such a 

 condition was rendered possible only by a complete solution of the calcareous parts leaving on 

 one side an external and on the other an internal impression of these parts. Considering the 

 structure, it is seen (Plate 23, fig. 1) that the ambulacral plates of the peristome pass directly 

 into those of the corona, and the plates of the corona are as given in the description of the species. 



