MOLLUSCOIDEA. 365 



by a pair of deltidial plates leaving a large circular foramen ; 

 interiorly the diductor muscular impressions are rather strong 

 and flabellate, occupying the upper third of the valve which is 

 considerably thickened. Brachial valve flattened or but moder- 

 ately convex, often with a broad, ill-defined mesial sinus usually 

 occupied by a single strong plication, but sometimes by from 

 three to five smaller ones ; internally the valve is much thickened 

 in the cardinal region, the cardinal process is prominent, being 

 bilobed or trilobed, with the hinge-plates well developed on either 

 side, the crura originate in front of and between the bases of the 

 hinge-plates at a moderate distance apart, they are directed for- 

 ward and outward, each one giving origin to a crural process a 

 short distance from the hinge-plate, which points inward and 

 forward, the brachidium is formed by rather broad primary 

 lamellae which curve outward from the crural extremities and 

 describe an incomplete subcircle of nearly one-half the diameter 

 of the valve; anteriorly they are sharply recurved and returned 

 at a short distance above the main portions, being united by a 

 transverse band just in front of the crural processes; the main 

 portions of the loop are united to the median septum which 

 originates in front of the cardinal process, by a transverse plate 

 near the end of the septum and just in front of the middle of the 

 loop. Surface of the shell marked by numerous strong concentric 

 lines of growth crossing the plications. Shell structure strongly 

 and minutely punctate, the punctae arranged in quincunx. 



Remarks. Specimens in which the brachidium of this species 

 can be observed are not common, but those which have been 

 observed show that this structure varies considerably in the pro- 

 portions of its different parts. The species is especially charac- 

 teristic of the Navesink marl where it sometimes occurs in great 

 numbers, and has not been observed in any other formation. It 

 is not closely related to any other American form except T. 

 vanuxemi from which it can be easily distinguished by its strongly 

 angular plications. 



Formation and locality. Navesink marl, Atlantic Highlands 

 (108), Middletown (ii3 2 ), near Oak Hill (121), near Craw- 

 ford's Corner (i26 7 ), near Holmdel (i28 3 , I28 5 , 127, 194), 



