140 Carl O. Dunbar, 



Trematospira bella, n. sp. 



Plate IV, figs. 4, 5 



Description: She'.l small, gibbous. Hinge-line shorter than 

 the width of the shell, cardinal extremities rounded. Surface 

 very strongly plicate. Valves subequally convex. The ventral 

 valve has a deep sinus bordered by a very high, narrowly rounded 

 plication on either side. Two smaller slender plications occupy 

 the sinus, and there is an additional sharp plication on each lat- 

 eral slope. The sinus is continued into a linguiform extension 

 that is strongly arched up almost at right angles to the plane 

 of the valve. The ventral beak is perforated by a small rounded 

 foramen but is tightly incurved over that of the opposite valve. 

 The dorsal valve has a high median fold occupied by three nar- 

 row plications, and each lateral slope is occupied by two high, 

 sharply rounded plications. A few lamellose lines of growth are 

 prominent near the anterior margin of the shell. 



Dimensions: Length, 10 mm.; width, 13.5 mm.; thickness, 

 10 mm. 



Discussion: This little shell very closely resembles T. gibbosa 

 of the Hamilton, from which it differs in being smaller and in 

 having only two plications on each lateral slope, while that spe- 

 cies has three or four. The ventral beak is also more tightly 

 incurved. These shells are so much alike that we can not avoid 

 the conclusion that the new species is the ancestor of the Middle 

 Devonian one. 



The little form figured in Volume 8, Part H, of the Paleon- 

 tology of New York (PI. 83, figs. 21-23) 3-"^ named T. tennes- 

 seensis, is of the same size as our species, but it has a different 

 aspect. It has but one plication in the sinus and two on the fold, 

 while the lateral plications are broader and blunter. Hall and 

 Clarke's species has, in fact, no conspicuous fold and sinus, and in 

 the profile view does not present the truncated appearance of 

 the anterior margin which characterizes the new species. Its 

 hinge-line is also not so straight and the ventral beak is more 

 erect, exposing the deltidium. 



Occurrence: The holotype was found in the Birdsong shale 

 at the old Swayne's mill locality near the mouth of Big Sandy 

 River, Henry County. 



