•14 PALEONTOLOGY OF NEW-YORK. 



tpirifer ditjuncttu i Datidson, Quar. Jour. Gteol. Society, Vol. 9, pa. 354, pi. 15, f. 1 - 6. 1853. 

 •• vtrntuilii : M'CoT, Brit. Pal. Fossils, p. 376. 1862. 

 •< disjuneiiu : MunOHisoii, Siluria, 2d edition, p,299, f.4. 1859. 

 " barumemu: Soweebt, MSS.i Salter, Jour. Gcol. Society, Vol.19, p. 480. 

 Mpirifera ditjuncta: Davidson, Monograph Brit. Dev. Brachiopoda, p. 28, 1864. 

 Deltkyrit perlata : CoNRAD, Ann. Rep. on Palasontology of New- York, p. 54. 1841. 



ckemungemu : Conrad, Jour. Acad. Nat. Sciences of Philadelphia, Vol. 8, p. 263. 1842. 

 . ditjuncta f Philups; Hall, Geol. Rep. 4th Dist. New-Tork, p. 269, f.8. 1848. 

 cutpidata : Hall (not Martin), lb., p. 270, f. 1. 1843. 

 acanthota: Id. lb., p. 270, f.2, 2 a. 1843. 

 \nermU : Id. lb., p. 271, f.4, 4 a; p. 270. 1843. 

 Compare Spiri/er whitneyi, Hall, Geology of Iowa, pa. 502, pl_. 4, f. 2. 1859. 



Shell variable in shape, seinioval, semicircular or subrhomboidal : hinge- 

 line usually as long as the width of the shell (rarely shorter), with the 

 cardinal angles slightly rounded, moderately or extremely extended 

 into long attenuated or mucronate points. Valves convex, or more or 

 less gibbous : entire surface, including mesial fold and sinus, plicated. 



Tbntral valve more or less convey, usually with a regular convexity on 

 the sides, but sometimes concave between the margins of the sinus'and 

 the extremities. The sinus is more or less deeply concave, rarely quite 

 shallow, usually rounded but sometimes angular in the bottom, and 

 the margins clearly defined : beak elevated and produced, moderately 

 incurved over the area, which is flat or concave and of variable height. 

 The fissure is often partially closed by an arching pseudo-deltidium. 



Dorsal valve moderately convex, sometimes a little concave towards the 

 cardinal angles : mesial fold distinctly defined, usually of moderate 

 elevation and convexity, sometimes subangular, and often divided by 

 a furrow down the centre : area very narrow. 



ScRFACE marked by from twenty to forty or more simple rounded or 

 subangular ribs on each side, with nearly equal interspaces. The mesial 

 fold and sinus are each marked by about ten, sometimes twelve or 

 fourteen ribs near the front, which proceed from the bifurcation of two 

 or three at the apex of each valve*. When the shell is perfectly pre- 

 served, the entire surface is marked by fine contiguous concentric 

 striae. 



• Mr. Davidson says the increase is by intercalation ; but strictly this is rarely true of our specimena. 



