INVERTEBRATE PALEONTOLOGY. 465 



P I a c e n t i c e r as placenta, Dekay isp.). 

 Plate 24, figs. 2, a, b. 



Ammonites placenta, I lekaj I 1828 >. Ann. X. Y. Lyceum of Nat. Hist., II, 278, pi. v, fig. 2 (3 by mistake). — 

 Morton (1829), jour. Acad. Nat. Sci. Philad., VI, 195 ; and Am. Jour. Sci. and Arts, 

 XVIII, pi. 2, tigs. 1,2, and 3 ; also (1834) Synop. Org. Rem. Cret. Formation U. S., 36, 

 pi. ii, tigs. I and 2. — Gabb (1861), Synop. Moll. Cret. Form., 15 (not A. placenta, Leek- 

 enby ( 1858), .lour. Geol. Soc. Loud.. XV. 10, pi. '2, tig. 1). 



Shell lenticular in form, attaining a large size; umbilicus small; volu- 

 tions deeply embracing, compressed laterally, with sides converging from 

 near the umbilicus to the periphery, which is very narrowly truncated and 

 flattened, or a little concave, with its smooth margins becoming more obtuse 

 with age; aperture narrowly sagittate; surface generally nearly smooth, or 

 only showing very obscure traces of curved, transversely-elongated promi- 

 nences on each side, with sometimes a row of very small indistinct nodes 

 around near the umbilicus — in young exfoliated shells, also usually showing 

 small, faintly-defined, divaricating corrugations, directed backward around the 

 outer half of eacli side.* 



Large examples attain two feet or more in their greatest diameter. 

 Young specimens, rf.70 inches in breadth, show a thickness of about 0.90 

 inch : while large individuals are proportionally thicker, and, on the periphery, 

 become more obtuse. 



Septa with twelve lateral lobes and as many sinuses on each side, in 

 large examples crowded and very complex; as made out from a specimen 

 measuring 6.50 inches in its dorso-ventral diameter at the point crossed by 

 the suture illustrated by the annexed cut, presenting the following characters: 

 Si phonal lobe nearly one-third wider than long, and provided with two prin- 

 cipal branches on each side, the two terminal of which are much larger than 

 the others, widely separated, diverging, and divided into four or five short, 

 palmately-spreading, sharply digitate branchlets, of which the three at the 

 end are usually so arranged as to give the extremity a trifid appearance; 

 first lateral sinus as long as the siphonal lobe, but narrower, with a very slen- 

 der body, and two very large, slender, deeply-divided, unequal terminal 

 branches, with deeply and irregularly sinuous margins, while its narrow, 

 zigzag body also supports several unequal, alternating, lateral branchlets and 

 smaller projections; first lateral lobe about as long as the siphonal, but much 

 narrower, oblique, and bearing two unequal bipartite, or tripartite, and digi- 



* These last-mentioned markings are made to look too much like sharp lines on fig. 2, a, pi. 24. 



59 n 



