ACKOTRETIDiE. 721 



diverging furrows, passing forward from the beak or internal foraminal opening, in broad curves which converge toward 

 the anterior margin but without meeting. These furrows inclose a thickened and somewhat elevated central area, 

 which, in the subumbonal region, is apparently free, projecting for a short distance, like a narrow, triangular shelf, 

 beneath which the foramen probably opened. The interior opening of the foramen is, however, not apparent on 

 any of the specimens examined, for, as usually preserved, the matrix has adhered to this subapical cavity, and in 

 a single example only is the shelf-like character of the median area distinctly demonstrated. A faint longitudinal 

 ridge passes from the apex of the shelf to the anterior margin, but no other markings are discernible on the interior- 

 except faint radiating or slightly undulating, probably vascular lines. 



The interior of the brachial valve, as far as known, shows no other characters than the radiating lines, which 

 appear to belong to the ornamentation of the outer surface. 



Shell substance tenuous, apparently corneous. External surface covered with more or less prominent, some- 

 times lamellose concentric growth lines, crossed by fine, gently cmved, radiating strise which are usually more promi- 

 nent when the concentric lines are exfoliated. 



Observations. — The interior of the ventral valve of this species is not unhke that of Acro- 

 thele. (Compare PL LXXXII, fig. 5a, with PI. LX, figs. If and 2.) It is not like that of 

 .Acrotreta sagittalis taconica (PI. LXXI). I have not been able to find an apical shelf in the 

 type specimens or other specimens. The material representing D. gulielmi is compressed in 

 the shale, so that the apical callosity is more or less compressed and distorted. The dorsal 

 valve, with its eccentric apex, is unhke Acrothele or any other of the genera of the Acrotretidse: 



With our present information, Disdnopsis is referred to the Acrotretidee and nearest to 

 Acrothyra. 



I have not discussed the description and conclusions of Matthew [1886, p. 37], as neither 

 Hall and Clarke nor I could find the muscle scars described and illustrated by him. The 

 student should carefully read Matthew's description and observations, as there is much oppor- 

 tunity for difference of opinion of minute shells compressed in shale. 



Formation and locality. — Middle Cambrian: (301g [Matthew, 1886, p. 39]) Sandstones of Division Ic of Mat- 

 thew; (308h) shales of Division Icl; and (301w) shales of Division lc2; all at Portland (now part of the city of St. 

 John), St. John County, New Brunswick. 



DisciNOPsis? suLCATus Walcott. 

 - Plate LXXXII, figure 6. 



Craniella f? sp., Walcott, 1905, Proc. U. S. Nat. Mus., vol. 29, pp. 4 and 6. (Listed.) 



Disdnopsis sulcatus Walcott, 1906, idem, vol. 30, pp. 568-569. (Desciibed and discussed as below as a new species.) 



This species is based upon the cast of the interior of a small ventral valve that in its 

 interior markings closely approaches the interior of the ventral valve of Disdnopsis gulielmi 

 (Matthew). 



The interior cast shows that the ventral valve was subcircular in outKne, moderately 

 convex, and with the apex probably perforated by a small, circular, foraminal aperture. In 

 front of the cast of the base of the foraminal aperture there is a broad depression that extends 

 to the front margin; on each side of the central depression an elongate, slightly depressed 

 area extends forward and outward from near the base of the cast of the foraminal aperture, 

 along the ridge on each side of the median depression; back of the base of the foraminal aper- 

 ture there is a narrow, short, arched furrow that indicates the presence of a corresponding 

 ridge on the interior of the shell. No other markings are shown on the cast, except the faint 

 outhne of what may have been the visceral area, on the median hne in front of the base of 

 the foraminal aperture and between the broad vascular sinuses. 



Observations. — This species is referred to the genus Disdnopsis as the result of comparison 

 with specimens of the interior of a ventral valve of D. gulielmi (Matthew). One interior of 

 the latter species has scars much hke those shown in D.9 sulcatus. 



This form has a sulcate ventral valve, hence the specific name. 



Formation and locality. — TJpper Cambrian: (C56) Lower part of the Chaumitien limestone, 25 feet below 

 the top of Pagoda Hill [Blackwelder, 1907a, p. 42 (part of last list of fossils)], 1 mile (1.6 km.) west of Tsinan, 

 Shantung, China. 



62667°— vol 51, ft 1—12 46 



