SIPHONOTRETID^. 629 



Orbicella buchi d'Orbigny (in part), 1850, Prodrome de Pal^ontologie, vol. 1, p. 20. (Changes generic reference, but 



includes "Orhicula reversa" also as a synonym of "Orbicella buchi") 

 Discina buchii (de Verneuil), Eichwald (in part), 1860, Lethaea rossica, ancienne p6riode, vol. 1, sec. 2, p. 914. 



(Described and discussed in French. Eichwald includes Jeremejew's Siphonotreta ladogensis as a synonym.) 

 Keyserlingia buchi (de Verneuil), Pander, 1861, Bull. Acad. imp. sci. St.-Petersbourg, tome 3, columns 46-48, PI. II, 



figs. la-h. (Described and discussed in German.) 

 Orbicella buchi (de Verneuil), Hall and Clarke, 1892, Eleventh Ann. Rept. State Geologist New York for 1891, p. 254, 



PL IV, figs. 8-10. (Mentioned as the type species. Figs. 8, 9, and 10 are copied from Pander, 1861, PL II, 



figs, la, Ih, and Ic, respectively.) 

 Keyserlingia buchi (de Verneuil), Hall and Clarke, 1892, Nat. Hist. New York, Paleontology, vol. 8, pt. 1, p. 118, 



PL IV, figs. 1-3. (Note fixing "K. buchi" as the type species. Figs. 1-3 are copied from Hall and Clarke, 



1892a, PL IV, figs. 9, 10, and 8, respectively.) 



Shell inequivalve, circular, or slightly oval longitudinally or transversely. Ventral valve 

 depressed conical with the apex from just in front of the posterior margin to one-fifth of the 

 length of the valve toward the front margin; a rouglily outlined false area is formed by the 

 incurving of the posterior slope from each side beneath the apex, to a narrow cleft that extends 

 downward from the apex a varying distance toward the posterior margin; the narrow cleft is 

 usually closed at the bottom, but in some examples it has a small elongate opening midway and 

 in others a minute rounded opening toward the lower or posterior part. The exterior of the 

 dorsal valve is slightly convex with the apex marginal. Surface marked by fine, concentric 

 striae and a few stronger lines of growth that pass beneath the apex across the false area except 

 where interrupted by the pedicle cleft ; when the latter is closed the strise do not cross the shell 

 substance at the bottom of the cleft. In addition there are faint traces of fine radiating striae 

 and on some ventral valves a few obscure, low, radiating ridges. 



The area of the ventral valve rises from the plane of the margins of the valve at an angle 

 of about 45°; it is slightly arched inward at the center, and is marked by strong striae of growth 

 parallel to the outer margin. Within the valve a median ridge of varying strength and length 

 extends forward from the front margin of the area. In fifteen examples it is less than one-half 

 the distance to the front margin, and in only one it extends forward beyond the center. On all 

 unbroken specimens a strong projection extends upward from the median ridge just in advance 

 of the area; it is narrow at the base, widening sideways and backward; it has a large variously 

 shaped anterior opening that narrows toward the base of the projection; the size of the projec- 

 tion and opening depends upon the amount of shelly matter that has been deposited on and in it. 

 The opening is the inner end of the pedicle tube; in some examples it is entirely filled with 

 shelly deposit (PL LXXXI, fig. 4b), and in others it is still open; in several examples with an 

 open interior aperture the pedicle cleft is closed at the exterior end. The median ridge, in front 

 of the pedicle tube, has a vesicular interior that gives it a hollow appearance when the top is 

 broken away. In several examples the pedicle muscle scar is clearly preserved, on the projec- 

 tion from the median ridge, just in front of the opening of the pedicle tube. On each side of 

 the median ridge close to the area the cardinal muscle scars are either in a depressed pit or on a 

 more or less elevated projecting growth of shell rising from the bottom of the valve; sometimes 

 the projections extend beyond the plane of the margins of the valve (PL LXXXI, fig. 4c), and 

 often the central projection about the pedicle tube extends above the plane of the margin. On 

 some shells the area indicated as the cardinal scar shows the points of attachment of tliree 

 muscles, probably the umbonal, transmedian, and anterior laterals; the only indication of the 

 points of attachment of the muscles corresponding to the central, outside, and middle laterals 

 of Obolus is on the sides and beside the elevated pedicle tube. 



The area of the dorsal valve has a broad, triangular, slightly concave space at the center 

 that is bounded by narrow flexure lines ; on each outer side of the flexure line the area narrows 

 and slopes upward at an angle of about 30° to 45°; it is marked by distinct lines of growth that 

 cross the central depressed space subparallel to the margin of the area. Within the valve a 

 strong median septum extends from the area to the front part of the valve; this septum when 

 broken shows an interior vesicular structure that gives the appearance of its having been hollow. 

 WeU toward the front an oval muscle scar (central) occurs on each side of the septum; the car- 

 dinal muscle scars are oval in outline close to the area and surrounded by a more or less thickened 



