BRACHYTHYRIS 379 



BRACHYTHYRIS ALTONENSIS n. sp. 

 Plate LIX, Figs. 7-14 



Description. Shell small, subglobular in form, a little wider than long, 

 the greatest width near the mid-length of the shell, the length of the 

 hinge-line equal to about one-half the total width of the shell, the cardinal 

 extremities rounded. The dimensions of two individuals are : length of 

 pedicle valve 14.8 mm. and 14.5 mm., length of brachial valve 11 mm. 

 and 10.8 mm., width 15.2 mm. and 15.5 mm., thickness 10.5 mm. and 9.8 

 mm., length of hinge-line 8.5 mm. and 9 mm., height of cardinal area 3 

 mm. and 2.8 mm. 



Pedicle valve strongly convex, the greatest convexity posterior to the 

 middle, the convexity extending out to the cardinal extremities, umbo 

 prominent, projecting posteriorly beyond the hinge-line, the surface 

 sloping abruptly to the cardinal margin, more gently to the antero-lateral 

 margins ; beak pointed and incurved ; cardinal area small, arched, broadly 

 triangular in outline, the lateral margins defined, although the demarca- 

 tion between the area and the lateral slopes of the valve is only slightly 

 marked, sloping regularly from the beak to the cardinal extremities ; the 

 delthyrium large, its base occupying more than one-third the cardinal 

 margin of the area; lateral slopes of the valve each bearing five simple, 

 rounded plications which grow progressively smaller towards the car- 

 dinal extremities ; mesial sinus distinct to the beak, rather shallow, 

 flattened in the bottom anteriorly, marked along the median line by a 

 narrow plication. 



Brachial valve much less convex than the pedicle, the greatest convexity 

 near or a little in front of the middle; mesial fold defined to the beak, 

 bounded by furrows which are deeper and broader than those between 

 the other plications, flattened on top and marked along the median line 

 by a narrow and shallow groove; lateral slopes most convex towards the 

 umbo, a little compressed towards the cardinal extremities, marked by 

 plications similar in form and number to those of the opposite valve. 



The minute surface markings of the shell have not been observed, but 

 concentric lines of growth of variable strength are present which be- 

 come more frequent towards the anterior margin. 



Internally the dental lamellae of the pedicle valve extend one-fifth or 

 a little more from the beak towards the anterior margin with slight 

 divergence, the muscular impressions are inconspicuous. 



Remarks. The types of this species are two nearly complete internal 

 casts from a dolomitic layer of the St. Louis limestone near Alton. In 

 recent collections the species occurs a few feet below the base of the 

 brecciated layer of the formation, and the types doubtless came from this 

 same position. If the specimens were differently preserved with the shell 



