448 Mr. A. Hancock on Vermiform Fossils. 



the latter ; only they are rarely so complicated. There are four 

 kinds of these fossil tracks. 



The first (PL XVIII. c, c, c) is in the form of a simple furrow, 

 with a narrow ridge on each side ; it is small, measuring scarcely 

 more than one-eighth of an inch wide, and has a wandering, 

 undulating course, which, however, is never very much compli- 

 cated, and is confined to the same horizontal plane. 



The second, which is a little wider than the above, is smooth, 

 cylindrical, and tortuous, and usually remains in relief on the 

 lower slab, but occasionally dips a little beneath the surface. 



The third (Pis. XVI. & XVII.) is irregularly cylindrical, 

 and though sometimes much undulated, is never convoluted or 

 very complicated; it is not strictly confined to the same hori- 

 zontal plane, but frequently sinks a little below the surface. 

 Full-sized specimens are half an inch wide, and are composed of 

 a series of nodules (a, a), which give them an articulated ap- 

 pearance. The nodules, which vary a little in size, are usually 

 about half an inch long, and are not very symmetrically or regu- 

 larly formed ; consequently the surface has generally an imper- 

 fect or worn character. The extremities are not often seen; 

 they are abruptly and irregularly rounded. This is the form 

 more particularly described by Mr. Wood, who has in his pos- 

 session the fine specimen figured in PI. XVI., which measures 

 nearly eight feet in length, though neither extremity is perfect. 



The fourth form (PI. XVIII. a), which seems to be the domi- 

 nant one in Northumberland, and is that figured and de- 

 scribed by Mr. John Dixon, is, as far as I have been able to 

 ascertain, usually in relief upon the upper surface of the lower 

 slab, from which it never deviates ; it is considerably depressed, 

 grooved (b) or ridged (PI. XIX. fig. 1 a) along the middle 

 line, and is occasionally very long. The specimen figured in 

 PI. XVIII. measures upwards of six feet in length, but was 

 probably much longer, for the extremities are not present. In 

 fact, it has not yet been determined to what length these 

 fossils extend, as none hitherto measured have had both ends 

 perfect. Large specimens are frequently an inch wide, and 

 generally much undulated, and occasionally, but never intri- 

 cately, convoluted. Such have the surface most frequently marked 

 with numerous, regular, rather fine, transverse striae or arched 

 ridges (PL XIX. fig. 2), which are sometimes very delicate and 

 close-set, though they vary in these particulars, and are occa- 

 sionally coarse and irregular, giving to the whole surface a 

 wrinkled appearance. Small specimens (fig. 1), on the contrary, 

 are frequently wound up in an intricate manner, the folds being 

 well rounded and often convoluted; but the surface is never 

 marked with transverse lines or wrinkles. 



