338 



ROBERT TRACY JACKSON ON ECHINI. 



ter through the mid-zone. Another specimen witli plates all in place (Plate 46, fig. 4), in 

 Mr. F. Springer's collection, measures 91 mm. in height, and 91 mm. in diameter through the 



mid-zone; the ambulacra at the 

 mid-zone are a little more than 10 

 mm., and the interambulacra are 

 about 44 mm. in width. 



The original description was 

 based on an internal mold, and as 

 the external characters differ con- 

 siderably from the internal, certain 

 features given as specific need modi- 

 fication, and both internal and ex- 

 ternal aspects have to be taken into 

 account (p. 60). 



The ambulacra are relatively 

 narrow, composed of four columns 

 of plates at the mid-zone. As seen 

 fi'om the exterior, the demi-plates 

 are narrow, the occluded wide, with 

 pore-pairs in that portion of each 

 plate nearest the next adjacent in- 

 terambulacrum (Plate 42, figs. 2, 3; 

 Plate 43, fig. 3). The occluded 

 plates are arched up in an elevated 

 median ridge which is formed 

 wholly by the thickening of plates, 

 no corresponding arch existing be- 

 neath them. The occluded plates 

 are produced as tongues laterally, which fit between corresponding tongues extending inward 

 of the adjacent demi-plates. Occluded plates bear two rows of alternating small secondary 

 tubercles, the margins of the plates curving to correspond to the positions of the same. 

 A single tubercle exists on the inner tongue of the demi-plates. While this is the character of 

 plates at the mid-zone and throughout the greater part of the test, vcntrally the plates built 

 in youth, and young plates dorsally near the apical disc are primai'ies, or otherwise simple 

 (Plate 42, figs. 1, 3), as later described in detail. Four or five demi-plates equal the height of 

 an adradial, and demi-plates on the marginal suture bevel over the adradials. Such is the 

 structure on the exterior, but the interior differs markedly. The interior of the ambulacrum 



244 



Text-fig. 244. — Looenechinns missouriensis (Jackson). Burlington 

 Group, Lower Carboniferous, Webb City, Missouri, Museum of Com- 

 parative Zoology Coll., 3,078 (from R. T. J. Coll.), holotype. Natural 

 size. Four columns of ambulacral plates, demi- and occluded; of about 

 equal width. Demi-plates ojiposite horizontal interambulacral sutures 

 fan-shaped, pore-pairs in middle of half-areas. In area A a sixth column 

 is represented by three plates 6, 6' and the next succeeding aboral plate. 

 .\dambulacral plates rounded on the adradial suture. The smaller figure 

 (a) shows an enlargement, X 2, of ambulacral detail. 



