FOSSILS. 603 



This shell resembles the T. machceriformis, but the anterior end is 

 proportionally longer and more regularly round, the posterior narrower 

 and more attenuated, and the convexity of the shell much greater. It 

 is much smaller and proportionally more elongated than the T. nasuta 

 of the Trenton limestone. Arisaig, coll. J. W. D. 



TelUnomya angustata, Hall. Shell elongate, narrow elliptical, more 

 than twice as long as wide, beaks fully one-third from the anterior 

 end. The anterior and posterior ends similar and equally rounded ; 

 basal margin regularly curved without indentation or sinuosity. Sur- 

 face evenly convex and very finely concentrically striated. Arisaig,. 

 coll. J. AV. D. 



Leptodomus [Sangidnolites) aratiis, Hall. Shell rhomboid-ovate, 

 ventricose, beaks at the anterior third of the valve, incurved and 

 pointed forward, umbones gibbous, a slight depression from the 

 umbo directly to the base of the shell, leaving a slight impression 

 in the central margin ; posterior slope sub-angular, the angle not 

 defined ; anterior slope with a defined angular ridge which borders 

 a large cordiform lunette ; anterior sharply rounded ; basal margin 

 nearly parallel w4th the hinge-line, curving upwards at the posterior 

 extremity, and somewhat obliquely truncated from the cardinal line. 

 Cardinal line straight posteriorly, with a wide and deep ligamental 

 area. Surface marked by strong unequal ridges and furrows parallel 

 to the basal margin, which become obsolescent on the posterior car- 

 dinal slope. 



It is scarcely possible to refer any fossil with satisfaction to the 

 genera Sangu'molites or Leptodomus of M'Coy, since the grouping of 

 species under these names appears to us to comprise a heterogeneous 

 assemblage in either case. Our shell corresponds in its external 

 features with Leptodomus costellatus of M'Coy, so far as the general 

 form, surface markings, ligamental area, etc., and is doubtless generi- 

 cally identical with that shell. Arisaig, coll. J. W. D. 



Megambonia (?) cancellata, Hall (Fig. 209). Shell sub-ovate, 

 widening posteriorly ; beak anterior, incurved, umbo gibbous, with 

 a gibbous umbonial slope on the posterior side, which is scarcely 

 diverging from the cardinal line ; posterior extremity rounded, the 

 basal margin arcuate, with a slight impression anterior to the middle, 

 the anterior end a little gibbous. Surface cancellated by concentric 

 and radiating elevated striie. 



It is not possible from the specimen before me to refer the species 

 satisfactorily to any known genus. Arisaig, coll. J. AV. D. 



Megambonia striata, Hall. Shell somewhat oval, the basal and 

 cardinal lines nearly parallel ; beak sub-anterior, small ; umbones 



