488 ANNELIDA. 



produced, at the present day, by small Amphipod Crustaceans" 

 {Sulcator arenarius and Kroyera arenaria), which burrow immedi- 

 ately below the sand on the sea-shore, and give rise to the following 

 appearances : (i.) Large tracks, about 3~8ths of an inch wide, 

 slightly raised, ribbon-like in shape, with a median groove, often 

 intricate and convoluted, sometimes knotted, and several feet in 

 length; (2.) Narrow wedge-shaped furrows, 2-ioths of an inch 

 wide, winding capriciously and often abruptly over the surface ; 

 (3.) Nodulated or articulated tracks, consisting of a small furrow, 

 with a rounded ridge on one side. Mr Hancock showed that 

 tracks of these three kinds are actually produced by the above- 

 named small Crustaceans, which burrow beneath the sand, but a 

 short way below the surface, " the arch or tunnel thus formed par- 

 tially subsiding, as the creature moves forwards, and breaking along 

 the centre," thus giving rise to a median groove. There is no 

 doubt that the phenomena so carefully observed by Mr Hancock 

 throw considerable light upon the subject of the supposed Anne- 

 lide tracks of muddy and sandy sediments ; but there is room for 

 much hesitation before concluding that any of these tracks, in the 

 older rocks at any rate, were really formed by Crustaceans like the 

 living Sulcator arenarius. One ground for such hesitation need 

 alone be brought forward here — namely, that the so-called " Anne- 

 lide-tracks " of the older Palaeozoic rocks often occur in vast num- 

 bers, in finely-levigated deposits, and throughout a thickness of 

 sometimes hundreds of feet of strata, and that it is almost incon- 

 ceivable that traces of the makers should not have been detected 

 in the same beds, supposing them to have been formed by animals 

 which, like Crustaceans, have a skeleton highly susceptible of 

 preservation in the fossil condition. 



In the second place, there are good grounds for ascribing certain 

 forms of " tracks," which have often been regarded as Annelidous, 

 to the operation of Univalve Molluscs. It is well known that many 

 Gasteropods, in crawling over the surface of sand or mud, give rise 

 to sinuous or intercrossing trails, the general feature of which is the 

 presence of a median furrow or groove, which may or may not be 

 bordered by a series of lateral markings on each side. Thus, the 

 annexed engraving (fig. 351) shows the track of a common living 

 Gasteropod {Purpura lapillus) when moving over firm sand, and it will 

 be at once seen that the markings produced by this Univalve are 

 exceedingly similar to the supposed Annelide-trails upon which the 

 genus Crossofiodia has been founded, both consisting of a central 

 groove, bounded by lateral raised margins. While it may therefore 

 be regarded as very probable that some supposed worm-tracks have 

 been formed by Molluscs, it cannot be altogether overlooked that 

 we do not usually meet with the fossil shells of such Molluscs in 



