G6 BULLETIN OF THE 



ing the minute ovisac with it. The removal of the last three or four 

 whorls is best accomplished under a dissecting microscope, with the aid 

 of the long-handled instruments used by dentists. These are chisels 

 of various forms, bent at the ends so that they can be held horizontally; 

 and almost every desirable point can be purchased, or obtained by 

 grinding down those commonly used. 



Great care must be taken in grinding down specimens for trans- 

 parent sections not to get them too thin. After a certain thinness is 

 attained, the carbonate of lime is apt to crack in a very irregular 

 manner, obscuring the structure and seriously enhancing the difficulty 

 of making accurate observations. 



The siphon is hardly ever at all points in the same plane, so that the 

 best practice in grinding the first side is not to pass beyond the outer 

 limits of this organ. Then, when the specimen is turned and receinent- 

 ed to a clean slide, the reduction of the exposed portion may be carried 

 forward to any desired level. The whorls are very apt to break off or 

 loosen during this last process, and the delicate crystals of lime on the 

 edges to become fractured, and therefore in most cases it is better to 

 allow a number of the earlier whorls to remain attached to the ovisac 

 as a support for those which are to be examined. 



EMBRYO. 



It has been customary to consider that one of the differences existing 

 between Goniatites, Ammonites, and Nautilus was to be found in the 

 absence of a globular ovisac at the beginning of the whorls in the two 

 latter. This view is very plainly expressed by D'Orbigny, Guido and 

 Fridolin Sandberger,* and Barrande ; but how this mistake could have 

 been made among Ammonites it is difficult to understand, unless indeed 

 the specimens the last-named authors examined, Amm. Icevigatus and 

 comphnatus Rein., had really lost their ovisacs. 



* " Bei der Unterscheidung der Gattung Goniatites von Ammonites ist dieses Merk- 

 raal nicht unwesentlich. Es scbeint niimlich, class den Ammoniten einc in dieser ab- 

 gesclinlirten Kugelgestalt sich erhaltende Anfangsellc nicht ziikommc. Mehrere 

 wohlerhaltene Arten, unter anderen Ammonites kevigatus und complanatus Rein, aus den 

 untercn Oolith von Thurnau zeigten schon von der Anfangselle aus regelmassig kegel- 

 formiges Anwachsen ohne jene Abschnurang." — Dk Versltinerung von Nassau. G. und 

 F. Sandberger. Wiesbaden, 1S50-5G, p. 59. 



