PLATE LXXVI. 



(Figures 1 3a. 'J, .il-:;4 by It. 1'. WiinriKLD; 4-7, 20 by E. Emmons; 8, U-l'), 17, iV28 Ijy F. B. Mkkk; 10, 16, 18, I» 



l)y G. B Simpson.) 



Genus RKNSSEL^RIA, Hall. 



I'age 2.')+. 



liENSSELiKUI.V MUTABILIS, H:lll. 



Pigs. 1, 2. Dorsal and profile views of a ratliL'r larfji" and somewhat elongate shell. 



Fig. 3. An outline sketch showing the loop and its relative length. 



Fig. 3a. The loop enlarged to show its form in more detail, the elongate triangular expansion formed by 



the union of the descending lamelUe, and the median ridge along the line of coalescence of these 



))arts, produced upwardly and po.steriorly into a free exti'emity. 



Lower HeUlorberg gi'oup. Bc.i-raft's Muuiitahi, Columbia county, JYeio York. 



Genus TRIGERIA. Bayle. 



Page 265. 



Trigeria Portlandica, Billiiig.s. 



Figs. 4, 5. Dorsal and venti-al ^aews of the original specimen of Mensselaeria Porllitiidku. Billings, which 

 is tentatively referi'ed to the Genus TRinERiA. 



Lower Helderberg group. Square Lake, Maine. 



Trigeria Gaudryi, OEhlert. 



Figs. 6, 1. Dorsal and profile views of an internal cast, provisionally referred to this species ; showin.;- the 

 form of the shell, the fine and simple plication of the exterior and the cavities left by the 

 removal of the dental pla'es in the pedicle valve and the median septum in the brachial valve. 

 In fig 7 the convexity of the valves is not satisfactorily represented, the bi-achial valve being 

 too convex and the opposite valve not convex enough. 

 OrLskany sandstone. Cwmherland, Maryland. 



Genus RENSSEL^RIA, Hall. 



Page 254. 



Renssel^ria Marylandica, Hall. 



Enlarged view of the cardinal portion of the pedicle-valve; showing the foramcm and deltidial 



plates. 

 Dorsal view of a typical exterior ; showing the fine surface plication and the oval outline of the 



valves. 

 Dorsal view of another individual, less sharply plicated and having a less convex brachial valve. 

 The interior of a nearly complete pedicle-valve ; showing the deep and strong dental lamella? 



restino' upon, though not consolidated with the bottom of the valve, and the elongate muscular 



impressions. 

 13. Longitudinal section of the valves ; showing the loop in profile, its anterior extension and the ele- 

 vation of the crural apophyses ; also the depth of the dental plates in the pedicle-valve and the 



umbonal thickening of the shell. 

 The interior of the brachial valve ; sh'owing the medially divided hinge-plate and the loop with 



its long, acutely triangular anterior plate and median ridge with its short and fi-ee posterior 



extension. 

 Profile of the specimen represented in fig. 11 ; .showing the convexity of the valves. 

 The cardinal portion of the brachial valve ; showing the form of the hinge-iilate, the obsolete N-is- 



ceral foramen, and the anterior median division. 

 Fig. 19. The interior of the umbonal portion of a brachial valve of an old shell in which the hinge-plate is 



much thickened. The specimen is projected backward to show the inner extremity of the 



