280 PALAEONTOLOGY OF NEW YORK. 



is divided by three or four parallel finer vessels, which are given off from the 

 margin of the trunk-sinus at right angles. In the larger valve on the inner 

 side of the outer pair of trunk-sinuses, may be seen fine branches, originating 

 at sharp angles. Further, in this valve may sometimes be seen, between the 

 outer and inner, and also between the two inner trunk-sinuses, two finer radi- 

 ating veins. All these vascular sinuses appear to unite in this valve to form a 

 large lacune surrounding the area of muscular insertion. A ridge-like, elongate 

 callosity crossed by irregular furrows, follows the cardinal margin on the dorsal 

 valve." 



Type, Terehratula Archiaci, de Verneuil. 



Observations. Leaving out of consideration for the present the peculiar 

 structure of the brachidium, we find the other characters of the shell suffici- 

 ently distinct from those of Renssel^eria and its allies ; and these difierential 

 features are found in the general smoothness of the exterior,* the inflexion 

 of the cardinal and lateral margins of the valves, the prominence and sharp 

 delimitation of the diductor scars of the pedicle-valve, and the subcylindrical 

 elevation of the hinge-plate into a veritable cardinal process, in which all traces 

 of supporting lamellae are lost, and whose posterior face is grooved and striated 

 by the insertion of the muscles. This latter feature is unusual in the palaeozoic 

 terebratuloids, and is at once suggestive of the structure in the large and heavy 

 rhynchonelloids (Plethorhynchus), so that it proves a distinctive character of 

 importance. 



An excellent representative of this type of structure is the American Oris- 

 kany species, originally described f as Mega.nteris ovalis, Hall, but which has 

 been subsequently and currently referred to RENsSELiERiA. In the original 

 description the similarity of the internal casts to those of Megalanteris Archiaci 

 was noted. Megalanteris ovalis is not an abundant species in the Oriskany 

 sandstone of New York and Ontario, and is represented in the formations in 

 the vicinity of Jonesboro, Illinois, by a smaller, more elongate form with erect 

 beak, deep cardinal and marginal excavations, the Rensselaria Condnni, of 



* Internal casts fi-equently show a minutely radiate surface about the margins, but this structure prob- 

 ably beloiig-s to the inner laniinie of the shell. 



t Tenth Annual Report on the Condition of the N. Y. Stale Cabinet of Natural History, p. 101. 1857. 



