No. 89.J 



71 



OBSERVATIONS' ON THE GENUS AMBOCCELIA. 



In the Journal of the Academy of Natural Sciences of Philadelphia, Vol. viii, 

 1842, Mr. Conrad described, under the name Orthis umbonata, a fossil shell from 

 the shales of the Hamilton group at Moscow in New-York. This species is common 

 in the same group from Seneca lake to the shores of Lake Erie. There are likewise 

 other similar forms less abundant in the Hamilton group of New-York, and one 

 species in the Chemung group, and we know a single species from the Coal mea- 

 sures. The species all have the ventral valve extremely convex, arcuate and ven- 

 tricose; while the dorsal valve is nearly flat, or sometimes slightly concave or 

 convex. These species differ essentially from Orthis in external appearance and 

 internal structure; and from the ventricose form of the ventral valve, I have 

 proposed the generic designation Ambocoelia. 



GENUS AMBOCOELIA ( n.g.). 

 [ Gr. ajijScjv, umbo ; and KotAia, venter.'] 

 Generic characters. Shells bivalve, inequivalve, equilateral, plano-convex or 

 concavo-convex : valves articulating by teeth and sockets; cardinal line equal- 

 ling the greatest width of the shell; area common to both valves; foramen tri- 

 angular, extending also to the area of the dorsal valve. Dorsal valve flat, de- 

 pressed convex or concave; cardinal process bifurcate. The foveal plates are 

 straight, longitudinal, pointed at their inner extremities; and from their centre 

 on the outside extends a callosity curving around the dental sockets, which open 

 towards the cardinal line. Muscular impressions four, and distinctly marked in 

 the middle of the valve or below. Ventral valve arcuate, with or without a 

 distinct sinus : beak arching over the area; dental lamellae strong, extending in 

 short obtuse teeth. The impressions of the cardinal muscles form two semioval 

 spots near the middle of the concavity between the beak and base of the shell. 

 Surface very finely cancellated by obscure radiating striae and fine concentric 

 lines of growth. Shell-structure fibrous (or fibro-punctate) : lustre pearly. 



The typical species of this genus is the Orthis umbonata of Conrad. The Spiri- 

 fer unguiculus of Sowerbt may likewise be included in this genus; and also 

 Orthis subumbonata, O. preumbona and O. nucleus, as well as another species 

 in the Chemung group which I have heretofore referred to O. unguiculus ( S. 

 unguiculus of Sowerbt). There is likewise a Carboniferous species, Jj/. gemmula; 

 making five known American species, and one European. These together constitute 

 a very natural group, readily determined by external characters, and easily 

 separable by these alone from Orthis or any other genus of Brachiopods in the 

 American strata. 



In several of the species there is a distinct mesial sinus upon the ventral valve, 

 a feature strongly marked in j^. umbonata, while in the other Hamilton species 

 this character is less conspicuous; but it is seen in the Chemung species, and in 

 ,the European form j^. unguiculus as figured by Phillips in his Palaeozoic Fossils. 



j Fig. 1, 2, 3. Dorsal, ventral and profile views ot Amhocalia umbonata, natural size, from the 

 1 shales of the Hamilton group. 



