88 Prof. P. M. Duncan on Upper Silurian 



marked with irregular swellings, but the rest plain. Their 

 tips are slightly expanded and faintly frilled. The smaller 

 arm is nearly cylindrical, but more or less conical. 



Under transmitted light these spicules appear nearly homo- 

 geneous ; but polarized light enables minute granules, 

 crystals, and very rarely cleavage-planes to be seen ; never- 

 theless the refraction of the mineral is insufficient to produce 

 much colour on crossing the Nicols. 



The breadth of the arms is from -^ to -^ of an inch ; and 

 the length is from -^ to rather more than jhs inch. Some of 

 these spicules, whilst retaining their general typical form, 

 differ much in external aspect. They may have two arms 

 slightly bent, expanded at the end, and rough (not on the 

 sides forming the angle), and the third visible arm may be 

 stunted ; or the arms may be equal and straight, with a 

 fourth like a well-developed knob or a long shaft. 



Spicules from the canal-system are invariably joined with 

 their neighbours to form a latticework. They consist of 

 normal trifid forms, of trifid forms with short and large 

 third limbs, of four-limbed forms, the extra limb resembling 

 the others or longer or shorter. In some the fourth limb is 

 situated at the junction of the other three, and, being in a 

 plane at right angles to them, produces a general swelling at 

 the junction, so as to interfere with the angle of the union of 

 the limbs, it being occasionally replaced by a concavity or 

 even a slight swelling. The limbs are usually plain on one 

 side and roundedly dentate, or irregularly rounded and slightly 

 spinose. The junction of the spicules with their neighbours 

 may take place through the ends of the limbs uniting in a kind 

 of suture, or by a joining which leaves no indication of its 

 exact position. The skeleton thus formed is very irregular at 

 the outer part of the central space, and then, as the canal-sys- 

 tem commences, becomes very regular. The symmetrical 

 arrangement of the spicules of the sides of the canals is very 

 exact; and one canal is separated from its neighbours by 

 common latticed walls. The diameter of the canal-space is 

 greater than that of any of the openings in its walls ; and the 

 one is produced by four or five spiculate series extending 

 around a space ; and the others are the spaces left between the 

 arms of neighbouring spicules. 



The junction of the spicules is usually by the ends of the 

 limbs ; and the exceptions are rare ; but in the centre of the 

 body of the fossil instances are to be seen where the end of 

 the limb of one spicule is attached to the middle of the cylin- 

 drical limb of another. The method of normal junction of 

 different spicules is by the clasping and surrounding of a 



