58 



Again, by a comparison with the types (the only true resort in ease 

 of a dispute) I have ascertained the relations of Gryphaea Pitcherii 

 and Marcou's G. dilatata var. Tucumcarii. (See Proc. Acad. Feb. 

 12, 1861.) 



In preparing the synonymy of the foreign species, I have placed 

 most confidence in special monographs, preferring in all cases to defer 

 to the opinions of such authors as have the reputation of being most 

 reliable. 



As to the American species, in addition to my own acquaintance 

 with the subject, I have received much valuable assistance from my 

 friends, Messrs. Conrad and Meek, both in conversation and by means 

 of their numerous papers, more especially from the excellent catalogue 

 of the species from Nebraska, recently published by the latter gentle- 

 man in connection with Dr. Hayden. 



For convenience, I have used many abbreviations, a list of which 

 is given below. When two or three numbers follow a reference, un- 

 less otherwise stated, the first designates the page ; the second, the 

 plate; and the rest the figures, thus : " Con. Jour. Acad. v. ii, 275, 

 24, 3," means Conrad, Journal of the Academy, volume 2, page 275, 

 plate 24, figure 3. 



I have spared no labor to make this the most complete paper of 

 the kind ever yet published, and to that end have gone over the en- 

 tire subject anew; still a work like this must almost necessarily have 

 some imperfections, spite the utmost vigilance of the author. I shall 

 therefore feel myself under many obligations to any one who may 

 take the pains to point out to me any defects, and I shall endeavor at 

 a suitable time to correct them. W. M. G. 



LIST OF ABBREVIATIONS. 



d'Arc. M. G. S. F. — d'Archiac Memoirs of the Geological Society 

 of France, 2d ser. vol. ii. 



B. & C.— Bayle & Coquand, id. 2d ser. vol. iv, 1851. 



Baily, Q. J. G. S. — Quar. Jour. Geological Society. London, vol. ii. 



Blainv. — Blainville (H. D. de) Memoire sur les Belemnites, con- 

 siderees zoologiquement et Geologiquement. Paris, 1827. 



Von Buch, Petr. rec. par Ilumb. — Petrifications recueillies en 

 Amerique, par Alex. Humboldt et par Chas. Begenhardt. Ber- 

 lin, 1839. 



(Jon. — Conrad (T. A.), Proceedings of the Academy, Philadelphia. 



