CENTRONELLOIDEA 247 



floor of the valve at either side of the median line, these septa continue 

 as crural lamellae anteriorly beyond the articulation of the valves and their 

 inner margins are continued into the crura. The brachidium is short, not 



o 



9900 



FIG. 23. A series of six cross-sections of the rostral portion of the shell of 

 Centronelloidea rowleyi (X 2^), showing the dental lamellae of the pedicle 

 valve, and the crural lamellae of the brachial valve which support the divided 

 hinge-plate, posteriorly. 



reaching beyond the middle of the shell, and like that of Dielasma in 

 form, but with much less widely diverging primary lamella?. 



Remarks. In its external form this genus is like Centronella, but its 

 brachidium is essentially like that of Dielasma, although the development 

 of the crural lamella? in the rostral portion of the brachial valve is entirely 

 different from that genus. The arrangement of these lamellae more nearly 

 resembles the condition in Dielasmoides, but the plates supporting the inner 

 margins of the lateral divisions of the hinge-plate continue further 

 anteriorly, forming a free V-shaped cavity beyond the articulation of the 

 valves, and the inner margins of the walls of this cavity give origin to 

 the crura. The external form of the shell is entirely different from either 

 Dielasma or Dielasmcides. 



CENTRONELLOIDEA ROWLEYI (Worthen) 

 Plate XXXI, Figs. 5-15 



1884. Terebratula rowleyi Worthen, Bull. No. 2, 111. State Mus Nat. Hist , 



p. 23. 

 1890. Terebratula rowleyi Worthen, Geol. Surv. 111., vol. 8, p. 102, pi. 11, 



figs. 6-6b. 

 1894. Terebratula rowleyi Keyes, Mo. Geol. Surv., vol. 5, p. 105, pi. 41, 



fig. 23. 

 1897. Dielasma ? rowleyi Schuchert, Synop. Am. Foss. Brach., p. 213. 



Description. Shell small, centronelliform, longer than wide, the greatest 

 width anterior to the mid-length of the shell, ovate-subcuneate in outline, 

 the postero-lateral margins a little concave and meeting at the beak in a 

 very acute angle, the antero-lateral and anterior margins regularly 

 rounded or the anterior margin somewhat truncate. The dimensions of 

 two nearly perfect specimens, metatypes, are : length of pedicle valve 10 

 mm. and 9.2 mm., length of brachial valve 9 mm. and 8 mm., greatest 

 width 7.7 mm. and 7.8 mm., thickness 5 mm. and 4.8 mm. 



Pedicle valve most convex posterior to the middle, narrowly rounded 

 transversely across the median line in the posterior portion, with the lat- 



