﻿Bibliography. 203 



na, and 30 (27 Polygastrica, 3 Phytolitharia) are identical with those 

 which at Caltasinetta undoubtedly belong to the chalk formation. 



5th. It appears that the exclusively American genera do not, as was at 

 first supposed, present only single species. The genus Goniothecium has 

 now 7 species, Rhizosolenia 2, and 4 species of a new genus, Dicladia, 

 have been detected. 



6th. The number of genera not yet found any where but in America 

 has been increased by this examination from 6 to 11, and it is probable 

 that the Triceratium spinosum of Bailey will form a twelfth, which may 

 be called Tetrachaeta, and which differs from Triceratium as Denticella 

 does from Biddulphia, but has 4 instead of 2 lateral spines. 



From Ehrenberg's account of the new American genera we have se- 

 lected the following as presenting the distinguishing characters of some 

 of the most interesting siliceous shelled Polygastrica. 



The four genera Asterolampra, Eupodiscus, Aulacodiscus, and Symbo- 

 lophora, belong to the family of Naviculte, and resemble Coscinodiscus in 

 their orbicular form and by not forming chains. They are distinguished 

 as follows : 



1. Asterolampra. Divided in the central portion alone into imper- 

 fect cells by slender septse, which do not reach the margin, alternating 

 with radii which reach the margin and which are not supported by septae. 



One very elegant species, A. Marylandica, with 8 marginal radii, oc- 

 curs at Piscataway, Md. 



2. Eupodiscus. Having tubular processes with perforated apices near 

 the margin of both sides. The name Eupodiscus is now substituted for 

 the old name Tripodiscus, which is rejected as inappropriate, as Ehren- 

 berg has by his own observations confirmed our statement that the num- 

 ber of processes may be greater than three. The species with 6 projec- 

 tions is called Eupodiscus Rogersii by Ehrenberg, while the one with 

 seven is called E. Baileyi. We still think that the number of the pro- 

 cesses will prove to be variable even in the same species. 



3. Aulacodiscus. Having (instead of the tubular pediform processes 

 of Eupodiscus) clefts radiating from the centre to tubercles near the mar- 

 gin, and with the surface granular, not cellular. One species, -A. crux, 



is found fossil at Richmond. 



4. Symbolophora. Having the septa? and cells radiating from a solid 

 angular centre ; surface not cellular. The only species, S. Trinilatis, 

 from Piscataway, is a very remarkable and elegant form. 



* As Ehrenberg in his tables indicates a doubt as to the tertiary character of our 

 infusorial deposits, it is proper to state for the information of those not familiar 

 with American geology, that the infusorial strata of Maryland and Virginia are 

 certainly of the epoch* of the miocene tertiary, as is shown by their containing 

 casts of miocene fossils and by their having strata of numerous and well pre- 

 served miocene fossils both below and above them. J. W. B. 



