[MATTHEW] STUDIES ON CAMBRIAN FAUNAS 117 



abruptly from the ventral limb and curves down slowly to the dorsal 

 limb ; on a few well preserved opercules, two very faint ridges diverge 

 from the nucleus toward the dorsal limb, extending about one-third of 

 the distance, and dividing the elevated portion of the opercule into three 

 nearly equal parts ; the middle of the dorsal limb shows very little 

 flattening on approaching the margin, but the lines of growth become 

 there more distinct ; on the ventral limb the striae of growth are still 

 more distinct, and this limb is usually somewhat bowed upward along the 

 middle. A groove, broadening toward the margin, extends from behind 

 the nucleuH on each side to the lateral angle. The operculum has a 

 diameter of 16 mm. 



When the ventral limb is broken off, owing to the smoothness of the 

 central area, this operculum is apt to be mistaken for an Obolelloid 

 brachiopod. 



The tubes of this species not infrequently contain those of smaller 

 individuals of the same species or of Orthotheca, and more rarely shells of 

 small gasteropods are found within them. 



In view of the antiquity of the fauna, of which H. excellens forms a 

 part, the changes which the shell underwent during growth are of interest 

 to the biologist, as they show that already the genus Hyolithes had passed 

 through some important changes of structure. 



We have already mentioned the fact that there was a transfer of a 

 portion of the dorsal side to the ventral surface during the growth, but 

 this transfer was not so entire as to make the transferred area invisible 

 from the dorsal side, for from that side it is slightly in view. Still it 

 is clear that this shell is to be classed in Holm's section Magnidor- 

 sati, the more advanced of the two groups into which Holm divides the 

 Hyolithes (sens, strict.). This shell then began its growth as an Equidor- 

 satus and ended as a Magnidorsatus, and this will probably be found to 

 have been the case with many of the Magnidorsati. It would appear 

 then that we have not reached the original fauna of Hyolithes in which 

 only Equidorsati are known. This is to be looked for further down in 

 the Etcheminian, or even perhaps in some older system of rocks. 



In its 3'ounger condition this species was characterized by a more 

 prominent lip and by more strongly arched striae on the dorsal side, than 

 at later stages of growth. The young, therefore, approached somewhat 

 nearer to Hyolithes Billingsi in form, and it is possible that there may be 

 a small species of Hyolithes of this form present in the limestone at 

 Smith Sound which the material on hand has not enabled us to discrimi- 

 nate. 



Operculum of young ? 



One fact which lends countenance to this idea is that an operculum 

 of small size occurs here, which differs in form from that of H. excellens. 



