PUGNOIDES 195 



The species has been referred to the genus Pugnax by Hall and Clarke, 

 but the internal structure of the brachial valve is that of Camarotcechia 

 rather than Pugnax. The specimen figured by these authors as the 

 brachial valve of the species with its broadly divided hinge-plate unsup- 

 ported by a median septum, must be either incomplete or it must be some- 

 thing else. Repeated sections of authentic examples from Pella, Iowa, 

 show the camerotcechoid characters here described and illustrated. 



Horizon. Ste. Genevieve limestone. 



PUGNOIDES BOONENSIS (Shumard) 

 Plate XXV, Figs.. 22-26 



1855. Rhynchonella Boonensis Shumard, Geol. Rep. Mo., p. 205, pi. C, 

 figs. 6a-b. 



1894. Rhynchonella boonensis Keyes, Mo. Geol. Surv., vol. 5, p. 101. 



1895. Leiorhynchus (?) Boonensis Hall and Clarke, Pal. N. Y., vol. 8, 



pt. 2, pi. 60, fig. 35. 



Description. Shell of about medium size, broadly ovate-subtriangular 

 in outline, wider than long, the greatest width near the mid-length, the 

 postero-lateral margins nearly straight and meeting in an obtuse angle, 

 the antero-lateral margins rounded, the anterior margin sinuate. The 

 dimensions of a nearly perfect specimen are: length of pedicle valve 

 19.1 mm., length of brachial valve 17.9 mm., greatest width 24 mm., 

 thickness 17 mm., width of mesial sinus of pedicle valve in front 17 mm. 



Pedicle valve gently convex in the umbonal region, flattened towards 

 the antero-lateral margins, strongly arched from beak to front along 

 the median line, the curve forming nearly a semicircle, narrowly but 

 rather abruptly curved along the postero-lateral margins and a little 

 inflected to the cardinal extremities; mesial sinus originating near the 

 mid length of the valve, becoming broad and rather deep anteriorly 

 and produced in a broadly rounded lingual extension whose surface lies 

 at nearly a right angle to the plane of the valve ; the beak obtusely pointed 

 and only a little incurved; plications obsolete upon the posterior portion 

 of the valve, but a little developed in front of the middle, those of the 

 sinus, two in number, originating near the mid-length of the valve, 

 rounded on top and becoming stronger towards the anterior margin, on 

 one or both sides of this pair of strong plications there is sometimes a 

 much fainter plication initiated near the margin; on each side of the 

 mesial sinus upon the lateral slopes one or two faint plications are com- 

 monly present which are limited to the extreme margin of the valve. 

 Internally the dental lamellae are short, between them in the rostral por- 

 tion of the valve is a short, low, median ridge, but anteriorly, beyond the 

 extremities of the lamelke the inner surface of the valve is somewhat 

 excavated for the muscular attachment. 



