46 G. F. MATTHEW: ILLUSTRATIONS OF 



great work ou the Pteropods of the Silurian System in Bohemia, aud uses the term 

 " grande face" for the longer side, and " petites faces" for the shorter side of the shell. 

 His terms, however, are artificial, as they imply a three-sided shell ; it is true that many 

 species of Hyolithes have a rudely triangular form, but others aire nearly cylindrical or 

 conical, and the flattening of the sides, even in those that are triangular, is more decided in 

 the enlarged part of the cone than toward the apex. The writer has refrained from using 

 Barrande's terms for the sides of Hyolithes, notwithstanding their convenience, for the Hyo- 

 lithoid shells have a decided bi-lateral symmetry, as Barrande himself has observed, and it 

 is desirable that we should ascertain which is the dorsal and which the ventral side of 

 these fossils. As a means of determining this point, we may compare them with other 

 classes of the Mollusca. 



Cephalopods. One rule for determining the ventral side of a mollusc, referred to by S. 

 P. Woodward,' viz., " that the under or ventral side of the body is that on which the 

 funnel is placed," is not without exception, for we have only to compare the animal of 

 Nautilus pompilius with that of any genus of the Dibranchiate Cephalopods to see that 

 the position of the funnel is not uniform. It is quite possible, however, that the position 

 of this organ in the living Nautili is anomalous, for if Barrande's interpretation of the 

 lobes of the orifice in Gomphoceras and Phragmoceras is correct, the normal position of 

 the siphon in these fossil genera is posterior or ventral. HoweA'er this may be, the 

 convex side of the spiral in the coiled Cephalopods is generally recognized as the dorsal. 

 If the Hyolithidœ be compared with such Cephalopods, the convex side of the spire in these 

 slightly curved shells may also be regarded as the dorsal, for the following reasons : — 



1. The striœ of growth advance on that side, and in most species it is the longer side. 



2. The nucleus of the operculum is found next the opposite side. 



This position of the operculum is parallelled with that of Ammonites, if, as is supposed, 

 the fossil called Aptyrhvs is the operculum of Ammonites. 



Gasteropods. Whether the Cephalopods furnish a parallel to the operculum in Hyo- 

 lithes or not, it is quite certain that many genera of Gasteropods do, and we shall 

 find that the nucleus of the operculum lies next the coluinella or ventral side of these 

 shells. As regards the form of the orifice, there is in the Gasteropods the same advance of 

 the stria; on the dorsal side as may be observed in the shell-bearing Cephalopods and on 

 the side which we take to be the dorsal side in Hyolithes and its allies. The comparison 

 is more readily made with the holostomatous gasteropods, but if allowance be made for 

 the often excessive distortion of the aperture in the carnivorous genera and the carrion- 

 feeders, the same advanced lip on the dorsal side may be observed in these. 



In the Atlautido? among the Heteropods as well as in lanthina, etc., the tentacles and 

 eyes are dorsal and anterior, aud the operculum posterior as in other Gasteropods. 



Pteropods. The pteropods, a class to which the Hyolithoid shells have been thought 

 to have the nearest affinities, exhibit the same position of the dorsal and ventral sides 

 relative to the aperture, as is found in the Dibranchiate Cephalopods and the Gasteropods. 

 The extension of the dorsal side is very marked in some of the Hyaleidœ, and is observ- 

 able in the genera which approach the Hyolithoid shells most nearly in form, such as 

 Cleodora, Balantium and Creseis. 



See JIaiiu;i1 of llic Mdllii.sca, l>y S. 1'. Woddwurd, i>. l.'iii, fddtnnte. 



