1S8 MATTHEW ON CAMBEIAN 



more difficult to decide how these objects are produced ; whether they are the petrified 

 remains of real worms, or only tracks of worms iu saud, bored aud then filled up pas- 

 sages, or finally worm casts of the intestinal canal, such as one can see everywhere on 

 the beach that Arenkola piscatonm inhabits. Such an animal cements together the sand 

 which it swallows, by the mucus of the intestinal canal ; so it may happen that the worm- 

 like excrement may thus be held together in the fossil state. Probably the worm from 

 which this fossil was produced was one whose way of life resembled that of Arenicola. 



From this description it is evident that Prof. Torrell at first regarded this track as 

 that of a worm ; subsequently when describing it under the name of Psammichnites, he 

 considered the possibility that it might have been made by a mollusc or crustacean. 



It is also clear that he includes in the description the casts of large size, similar to 

 those large tracks of worms which he found in the same sandstones. 



By comparing the figures ArenicoUtes (or Pmmndchintes) gigas, given by Torrell, it will 

 be seen that the fossil agrees in all essential particulars with that which I have herein 

 referred to his genus Psammichnites. It may, however, be remarked that the full-sized 

 figure of the species, given by Torrell (Plate XI, fig. 1/.-), represents the form as much more 

 convex than it usually is found. But there is so much variety in the configuration of the 

 surface of the cast of the track, that Torrell's figure only imperfectly represents its features. 

 These vary according to whether the track runs over the top of a wave-ridge in the sand, 

 or extends along a hollow between the ridges ; when traversing the ridges the animal 

 moA'ed more rapidly and formed a deep round fnrrow, set oflî'by a narrow sharp ridge on 

 each side, which is separated from the main furrow by a small groove (fig. \b.) Elsewhere 

 we find the central furrow represented by a nearly flat band with three small furrows, one 

 central aud the other two at the sides ; this track appears much narrower than the typical 

 track, but outside of the three furrows are lighter traces of another pair of furrows, 

 defining a slightly raised swelling, which mark the full limit of width of the track (fig. 

 near le.) This track has pittings along the centre, aud when the centre is raised, appears 

 to bear depressed irregular hollows along the side. 



The usual form of Psammichnites has two broad raised convex ridges with a narrow 

 channel between them ; when badly preserved, this channel seems to be wider and the 

 ridges narrower, owing to the channel being filled partly with mud. In some well pre 

 served tracks, the ridges are seen not to be continuous, but to be a series of long spindle- 

 shaped swellings, with the summits of the swellings about one and a half inches apart, 

 and with one end having a longer slope than the other (fig. Id.) In others the ridges are 

 traversed by shallow diagonal depressions turning inward aud backward from each side, 

 aud about one inch apart (fig. !«.) 



ARENICOLITES, Sailer. 



DIPLOCRATERION, Torrell. 



Bv\rrows of worms referable to this genus are common at several horizons of the St. 

 John group, aud notably in Division 2. Those of the Basal series are not so conspicuous 

 but have some interesting forms. Such casts of worm tubes were arranged by Salter 

 under the above name. Salter's description is as follows : — 



