PLATE LI. 



(Figures 1-7, 13-16, 18. 23-26, -28, '29, 31-41 by G. B. Simpson: 8. ii, 20-22 by J. M. Clarke; 17 by F. 1$. Meek; 19, 27, 



30 by E. Emmons; 10-12 copies.) 



Genus HUSTEDIA, gen. nov. 



Page 1-20. 

 HuSTEDIA MORMONI, MaiCOll. 

 Figs. 1-4. Ventral, dorsal, frontal and profile views of an average individual ; showing the coarse plication 

 of the surface. 

 An enlarged viesv of the exterior of the pedicle-valve. X 2. 



The opposite side of the same specimen ; showing the coalesced deltiiiial plates. X 2. 

 An enlai-geineni of the umltonal region ; shciwingthe coalesced deltidial plates, and the flattened 



cardinal slopes of the brachial valve. X 5. 

 A preparatiiin of the hinge-plate, viewed from in front ; showing the elevation of the large 



recurved median part, the crui al lulies, and the projection of the anterior ligulate process. 

 A protile view of the same specimen ; showing the great recurvature of the body of the plate, 

 the direction of the crural processes, and the extent of the anterior ligulate process. X 5. (c.) 

 Coal Measures. Near Kansas City, Missouri. 



Genus UNCINELLA, Waagen. 



Page 1-23. 



Uncinella typica, Waagen. 

 Figs. 10-12. Dorsal, profile and ventral views of a typical specimen ; showing the extei-ior characters. 

 (Waagen.) 



Permo-carboniferous. Salt- Range, India. 



Genus EUMETRIA, Hall. 



Page 11.1. 



Eumetkia Veuneuiliana, Hall. 



Figs. 13, 14. Doi-.sal and protile views of an unusually elongate shell. 



Figs. 15, 16 Similar view.s of an example which has the usual form of the species. 



Fig. 17. A dorsal view of the original siiecimen of the species, enlarged. 



In all the above specimens the deltidial plates are seen to be wholly coalesced, which is the nor- 

 mal condition at matui-ity. 



Fig. 18. A dorsal view of a small inilividual in which the median division-line between the deltidial plates 

 is still i-etained. X 3. This and the preceding figure represent the species as it occurs in the 

 limestone at this locality. 



Fig. 19. The detached brachidium, enlarged, and viewed from the ventral side ; showing the attachment 

 of the crura to the primary lamellse, the form of the loop, its long, straight bifurcate stem, and 

 the shape of the spii-al cones. X 2. (c.) 



Fig. 20. The interior of the cardinal region of articulated valves, viewed with the plane of the hinge hori- 

 zontal. This prepai-alion shows, in the background, the lai-ge foramen and below it the flattened 

 inner surface of the coalesced deltidial plates, which afford no evidence of a median suture. On 

 either side are the elongate teeth filling the equally elongate dental .sockets. The hinge-plate 

 consists of two parts, (a) the posterior jiortion which takes the form of a crescent, its horns lying 

 back upon the inner surface of the deltidial plates and the nmbonal slopes ; this is connected 

 laterally with the socket-walls and anteriorly with (ft), the anterior portion, which is tent-shaped 

 and consists of two deep and bread lateral lamella i-esting on the bottom of the valve, united 

 above by a deeply concave horizontal plate ; from the anterior angles formed by the union of 

 these phates arise the divergent and gi-e,atly elevated crural processes. In this figure the anterior 

 portion of this apparatus is considerably foi-eshortened. It will be observed that the specimen 

 shows no evidence of the delthyrial tube occurring in Retzia, Hu.stedia, etc. X 10. (c.) 



Fig. 21. The same preparation viewed with the pedicle-valve inclined upward; showing the length of the 

 anterior transverse jilate. X 5. 



Fig. 22. A view of the same specimen with the pedicle-valve inclined downward ; showing the elevation of 

 the crural plates and processes. X 5. 



St. Louis grouji. Spergen Bill, Indiana. 



Fig. 23. An internal cast of a brachial valve which retains a portion of the hinge-plate and shows the back- 

 ward projection of the posterior crescent. 



Fig. 24. A dorsal view of a specimen, drawn from the impression of a natural mould represented in fig. 25. 



Fig. 2.'). A mould of the extei-ior of a poi-lion of both valves. 



Fig. 26. The umbonal portion of the same specimen, enlarged ; showing the maximum development of the 

 foraminal tube, which is but a slightly introvei-ted lamina. The figure was also designed to 

 represent the separation along the hinge-line of the deltidial plates from the flattened cardinal 

 sui-faces of the bi-achial valve, b it by an error in the lithography this line has been made to 

 appear as a break continuous with a slight fi-acture on each side of the pedicle-valve. X 3. 

 St. Louis group. Cfreeiie county, Missouri. 



