Axial Canals of 8ponge Spicules, &c. Bij Prof. Duncan, 563 



Probably there are more acerate spicula in the deposit than 

 any other forms, and there are two kinds of them ; one the 

 common acerate, and the other a fusiformi-cyhndrical, bent more at 

 the ends than in the middle and bluntly pointed. The normal axial 

 canal is present in some, but it is abnormal in most of these 

 spicula. 



The first abnormal kind (Fig. 6) is ro o inch long and y^Vo^ inch 

 at its greatest breadth. The axial canal reaches the point and is 

 exceeding minute, but it soon widens out slightly, and then sud- 

 denly still more so, and nearly fills up the end of the spicule. 

 Then it narrows to the normal calibre and reaches close to the 

 other extremity of the spicule. In another spicule of the same 

 dimensions, the axial canal is normal at one end, and is very 

 slightly enlarged in that half of its course; but in the other it 

 increases gradually in size, and then rather suddenly diminishes, 

 forming a rounded end, out of which passes a delicate normal tube 

 for a short distance, and it does not reach the termination of the 

 spicule. 



A third kind is ^V i^ich in length, and is very slender. The 

 spicule, Fig. 6a, is open at both ends. At one end the axial canal 

 is large and gradually diminishes to the normal condition. At the 

 other end the axial canal is small and gradually enlarges in the 

 first fifth of the spicule, and then diminishes and becomes normal, 

 joining the similar condition of the canal at the other termination. 

 A fourth specimen of an acerate is of the dimensions of yo-o by 

 x^Vo- iiich (Fig. 6*) ; it is marked on the surface by erosion, by 

 pits and irregular eminences, and it is open at both ends. The 

 axial canal fills nearly the whole, except at one end where it is only 

 slightly enlarged and open. 



One of the most striking forms of spicula, and probably the 

 commonest in the deposit under consideration, is an acerate more 

 bent at the ends than midway, and which is fusiformi-cyhndrical 

 and rather bluntly pointed. 



It was the study of numerous specimens of this type, that led to 

 the belief that the remarkable enlargements of the axial canal were 

 subsequent to a normal condition, in which the tube was excessively 

 narrow and did not reach quite to the end of the spicule. 



A normal spicule is about ^^ inch long and t-qVo inch broad. 



The first example of a departure from the normal type of the 

 axial canal, is in a spicule of the same dimensions. Near to one end, 

 the width of the axial canal is slightly greater than normal, it then 

 suddenly increases to nearly the whole breadth of the spicule, and 

 occupies a quarter of the length, then it gradually diminishes in 

 width, tapering down to that of the normal canal. This great 

 enlargement occupies two-thirds of the whole spicule, and there is 

 only a short normal tube leading from it, and which ends in a 



