THE FAUNA OF THE ST. JOHN GROUP. 45 



v.— PTEROPODA. 



TTYOLITHES, Eichwald. 



The hyolithoid.pteropods of the St. John group differ from the ordinary type of this 

 genus in several respects, but chiefly in that all the species known are furnished with 

 septa near the apex of the tiibe. 



In the examination of these fossils the writer has had the assistance of Professor Alpheus 

 Hyatt of Boston, who has given much attention to the structure of the Cephalopods, and 

 is well qualified to judge of the relationship of the pteropods in question to that class of 

 Molluscs. Notwithstanding the remarkable resemblance of some of these chambered fos- 

 sils to the Cephalopods, Professor Hyatt is of the opinion that they are not of this class, but 

 are true Pteropods. Since these fossils were submitted to Prof. Hyatt, others haA'e been 

 found which exhibit a narrow and more or less flexible tube attached to the point of the 

 shell, an appendage which appears to be analogous to the embryonic or larval tube or 

 prosiphon of the Cephalopods, as described by several authors. It has not been found on 

 all the species. 



Dorsal and Ventral Sides. All of the Hyolithoid species thus far discovered in the St. 

 John group are chambered near the apex, but some have also a sheath divided by dia- 

 phragms on one side of the tube. These two types of structure and especially the latter 

 are not always easily recognized, owing to the delicacy of the parts and the imperfect con- 

 dition of preservation, but they so palpably suggest relationships with the Cephalopods, 

 that it seems desirable to compare them with that order of molluscs. To do so it is neces- 

 sary that we should arrive at an intelligent understanding of the position of the animal 

 of Hyolithes and its allies, relative to the shell. 



In looking into the views expressed by difiereut naturalists upon this jjoint, we find 

 a considerable diversity of opinion. The late J. W. Salter, pahpontologist of the Geological 

 Survey of Great Britain, if one may judge from his written descriptions of the Cambrian 

 and Silurian pteropods, held the view that the side which in most species terminates in 

 the long arched lip of the aperture is the ventral side ; and this also is the opinion of the 

 late E. Billings, palfeontologist of the Canadian Survey. There is something to counten- 

 ance this view, if the thecoid shells be regarded as straight cones with sides of nearly 

 equal length, as some of them are ; for the shorter side, being more rigid and strong, is 

 adapted to protect the vital parts of the animal. But if we regard the oblique position of 

 the aperture, and the tendency to an arched form so frequently exhibited by the tube or 

 sheath in this group of shells, we should be inclined to an opposite conclusion. 



Barraude, whose opportunities for observation were very great, must have found a 

 difficulty in arriving at a conclusion on this point, for he is quite non-committal ' in his 



' INI. Barrande, however, seems to incline to tiilve the same view of tlie relation of these sides of the shell as that 

 adopted by Mr. Salter, for he says (p. G7) : " Nous nous bornons à faire remarquer, que, si on considère la conformation 

 de la coquille des Pterotheea (PI. 15), qui i)arait en connexion très rapprochée avec celle de IlijoliOics, on reconnaît, que 

 la surface convexe correspond aux deux petites faces de la gaîne, et que la surface concave correspond à la grande 

 face de celle-ci. Par conséquent, si la surface convexe de la coquille est regardée comme dorsale, le bord dorsal du 

 molluscjue serait appliqué contre les deux petites faces et le bord ventral contre la frrande face de la jraîne. Par 

 analogie, il on serait de même dans ITijoIUIkh.'" 



