1887.] 381 [[Brooks. 



radiating from the hollow of the main shaft. The pin-shaped 

 spicules also occur on the inner surface of the sheaths, but here 

 they are smooth and without the bristles, and somewhat flattened 

 on their ends where they rest against the conchiolin tube. The 

 spicules forming the inner layer of the sheaths are almost wholly 

 fusiform, or of some slight variation from that shape. 



In the extremely young siphons, including the caecum, the sheaths 

 are made up of slender threads, placed in the same way as the 

 spicules of the older sheaths. 



There is no regular arrangement of the spicules except at such 

 points on the anterior portions of the funnels where the siphuncle 

 rested. Here they are placed at right angles to the surface of the 

 funnel. The same arrangement is found in the spicules forming 

 the disc-like bottom of the csecum. Here they are placed at right 

 angles to the inner surface of the apex of the shell. 



The spicules are made up of slender transparent sticks of cal- 

 careous matter, much smaller than the threads forming the young 

 sheaths but still similar to them. These transparent sticks are 

 held together in bundles by organic matter. These, when exam- 

 ined under a high power, have the appearance of bundles of fagots. 

 In the older sheaths the bundles are yellow, while in the extremely 

 young ones they are white. 



The spicules of the anterior portions of the sheaths where they 

 embrace the funnels, are much smaller than those of the posterior 

 portion and often appear as mere granules. At this point the 

 sheaths are marked with a series of ridges extending around their 

 entire surface. These ridges are much deeper and more pronounced 

 on the dorsal or inner side of the sheaths than on the ventral or 

 outer side. The ridges indicate a series of interruptions in the 

 growth of the sheaths and the commencement of the formation of 

 new septa. 



The sheaths of the older siphons commence as a dense layer sit- 

 uated on the posterior inner surface of a funnel. This dense layer 

 extends through the funnel for a short distance when it at once as- 

 sumes the open structure which is retained throughout its entire 

 length. 



There seems to be a well marked period in the growth of the 

 siphons when they first commence to form spicules, but this, as yet, 

 has not been fully determined. The formation of sheaths of the 

 young siphons will form the subject for another paper, when it has 

 been fully examined. 



