SKULL AND OTHEK BONES OF LOXOMMA AiLMANNI. 217 



bases of the longer ones ; and there are others of intermediate 

 different lengths. They pass in from the periphery at first, but 

 for a short distance, straight, but soon form undulating and then 

 zigzag curves, which continue to the end, where, in places, two 

 or more may be seen united. 



Each concavity on the undulating sides of the plicae answers 

 to a secondary offset of the pulp-cavity; and the dentine par- 

 tially surrounding these little bays is disposed as a secondary 

 toothlet, of which the bay is its particular pulp-cavity. 



If now with the aid of a quarter-inch object glass (Powell 

 and Lealand's) we look at the dilated end of one of the primary 

 prolongations of the central pulp-cavity, which serves as a pulp- 

 cavity to a toothlet, we see the tubules of the dentine radiating 

 from its margin through a series of finely arched lines towards 

 the crown and sides of the toothlet ; before, however, reaching 

 the outer borders of these, they pass into a dark granular-look- 

 ing layer, which is parallel to the crown and sides, and in some 

 toothlets double. This layer consists of black lines forming 

 a close network, the meshes of which are minute and look like 

 cells, giving this layer its black and granular aspect. It is 

 usually well defined on its external side ; but towards the pulp- 

 cavity it is in many parts gradually thinned away, and continued, 

 here and there, a good way into the tubular dentine ; in such 

 situations the lines often lose their dark colour, and resemble the 

 tubes of dentine, with which it is not difficult to observe that 

 they arc continuous. The dentinal tubules can here and there 

 be seen as black lines approaching the dark network ; some can 

 be observed to divide into two, as is common in human dentine. 

 Many, perhaps all, of the dentinal tubes are thus, as it were, 

 arrested in their straight course by the black layer. 



Beyond this is a narrower and lighter-coloured tract, which 

 forms the external boundary of the crown and sides of the tooth- 

 let ; in it numerous closely set straight lines or tubules, mostly 

 pale, but some black, and of the same size as those of the black 

 layer, are visible, passing out of that layer to the exterior sur- 

 face of the tooth. Thus the whole thickness of the crown of 

 the toothlet is composed of tubular and granular dentine. The 



