GENERA OF THE SUBORDER PENTAMEROIDEA 



157 



(Walcott). In the dorsal valve there is no cruralium 

 as Walcott has described and illustrated. Instead of 

 there being a continuous spoon-shaped plate supported 

 by lateral septa, there are two plates beneath the 

 brachiophores which converge medially but never unite. 

 Each plate is supported by two or more septa, the usual 

 number being two, one inner and one outer (see pi. 15, 

 figs. 7, 10). 



CLirkella is evidently very closely related to Syntro- 

 fhina and is characteristic of the Ozarkian. It is 

 widely distributed geographically, being known from 

 Montana, British Columbia, and Phillipsburg, Quebec. 



plex which is supported for nearly its entire length by 

 a septum which is short and thick; septum extending 

 for half the length of the valve. There are accessory 

 lateral septa up to four in number aiding in the 

 support of the spondylium. Between these are other 

 low radial ridges. 



Dorsal interior. — Notothyrial cavity deep; sockets 

 deep, at the posterior the brachiophore plates form a 

 small, deep, spoon-shaped muscle platform supported 

 at the back by one or two pairs of lateral septa. After 

 a short distance the platform, which is evidently for the 

 attachment of the diductors, ends, but the two lateral 



«> 







I 



Fig. 22. — Yangtzedla foloi (Martelli). Serial sections through the beak of a mature shell. The internal structure 

 allies this genus with Clarkella Walcott and proves conclusively that it has no relationships with Triflesia. The stippled areas 

 indicate adventitious shell substance which fills the umbonal chambers of both beaks. Such a filling is common in many 

 genera of spondylium-bearing shells. The shell sectioned was 19.3 mm. in length. All structures disappeared 8.5 mm. 

 from the beak. Distances from beak: 



1 — 1.4 mm. 

 2—2.5 

 3—3.2 

 4 — 4.5 



5 — 5.8 mm. 

 6 — 6.8 

 7—7.3 



Genus YANGTZEELLA Kolarova 1925 



PI. 15, figs. 24-26; t. figs. 22, 23 



Kolarova, Bull. Geol. Soc. China, vol. 4, 1925, p. 219, 

 pi. l,t. fig. 1. 



Genoholotype. — Triflecia foloi Martelli 1901, 

 Bull. Soc. Geol. Ital., vol. 20, fasc. 1, pp. 302-304, 

 pi. 4, figs. 13-22. 



Description. Exterior. — Outline subquadrate, 

 hinge-line straight, narrower than the total width of 

 the shell ; cardinal extremities rounded ; lateral profile 

 biconvex, the dorsal valve having the greater con- 

 vexity. Anterior commissure uniplicate ; ventral sul- 

 cus very deep, defined only on the front half of the 

 shell; dorsal fold pronounced only in the anterior 

 area. Ventral interarea longer than the dorsal, apsa- 

 cline; beak curved slightly and may or may not be 

 resorbed by pedicle pressure; delthyrium open. Dorsal 

 interarea curved, apsacline, beak strongly curved, umbo 

 inflated, notothyrium open. Surface nearly smooth, 

 marked by concentric growth-lines and distant lam- 

 ellae, and by faint radiating ridges. Test fibrous, 

 impunctate. 



Ventral interior. — Delthyrial cavity deep; teeth 

 strong ; dental plates thick, forming a spondylium sim- 



septa persist, each bearing a laterally directed shelf and 

 a rather long brachial process. 



Geologic range. — The only known species is 

 y. foloi (Martelli) of the ? Lower Ordovician of 

 China. 



Distinguishing characters. — The chief diag- 

 nostic characters of Yangtzeella are its nearly smooth 

 exterior, Triflesia-hke or syntrophioid outline and pro- 

 file, ventral spondylium simplex which may be sup- 

 ported by two or four accessory lateral septa, and in 

 the dorsal valve a spoon-shaped plate supported by two 

 primary lateral and two accessory lateral septa. It is 

 impossible to say whether or not this plate bore muscles. 



Discussion. — Yangtzeella shows several interesting 

 features. One of these is the great amount of adven- 

 titious shell substance deposited in the umbonal cavities 

 of the valves, chiefly in the ventral one. In Kolarova's 

 sections (his pi. 1, figs. 2-6) the ventral shell for about 

 3 mm. from the beak is wholly test, and the same is 

 true of the Yale specimens for 2.8 mm. from the 

 beak. However, the septa show clearly in the .idventi- 

 tious substance so that none of the shell anatomy is 

 obscured in the sections. 



The supporting plate beneath the ventral spondylium 

 is commonly split or fractured so that it appears to be 

 a duplex septum, and yet it is not precisely like the 



