SALTER LOWEE SILURIAN CRUSTACEA. 95 



That there were two instruments or styles used to produce the 

 stroke is clear from the shape of the indent. In very shallow 

 imprints the marks of the two prongs (set rather widely apart) are 

 visible along the whole length of the indent, and keep at an equal 

 distance along this space. When the indent deepens, the prongs 

 appear to have been pressed together as the stroke proceeded, for 

 the imprint is narrower at the deeper part ; and then, as the stroke 

 was ended, they did not in all cases spring open to their full width, 

 but remained half closed until the prongs were withdrawn. But at 

 the first impact they are always about two lines apart ; and, as 

 before stated, when the stroke has been a shallow one, this breadth 

 is preserved to the extremity of the imprint. 



The mud must evidently have been very soft when these impres- 

 sions were made ; for there seems to have been no resistance, save 

 that which the natural density of the substance would readily account 

 for. That two prongs were used in the operation is equally clear ; 

 and we must look, therefore, for a Crustacean which must have 

 possessed such a pair of caudal appendages. There is no probability 

 whatever of the markings having been produced by lateral feet, for 

 the stroke has in every instance been unique. 



The question next arises, what Crustacean could have formed such 

 a track ? We know of many large forms in the Upper Silurian rocks 

 which have strong prong-like tails ; and it is to these I should natu- 

 rally look for comparison, were it not that all the species of Ceratio- 

 caris known, besides being strictly Upper Silurian, have the median 

 prong, or telson, of the tail so strong and large that it alone would 

 have made the imprint. Such a creature might indeed easily drag its 

 long telson along with a wriggling motion, and produce the wavy, 

 zigzag trail observed on the slab of Normandy sandstone. The 

 wavy line is just what would be produced by a weapon trailed slowly 

 along a ridge of muddy silt, which detained it, while the intervening 

 hollows would not be even touched by the instrument. I refer that 

 track to Ceratiocaris, or an allied genus. But the double imprint 

 of our specimen is to be interpreted differently. Either we must 

 conceive a genus allied to Ceratiocaris, but with the median tail- 

 prong abortive and the two lateral ones only developed, or we must 

 look to rocks of a higher antiquity than the Llandeilo-nags for its 

 analogue. I have lately learned that the caudal appendages of 

 Hymenocaris, from the Lingula-flags of Wales, were composed of six 

 pieces, four short and two long ones (fig. 3, p. 90). If we have not 

 yet discovered the large Crustacean which made these broad and 

 vigorous imprints, we may feel pretty certain, from analogy with 

 forms found both on higher and lower horizons, that it was a great 

 Phyllopod, with a prong-like tail, intermediate (in all probability) 

 between the two genera above named. A single track becomes in 

 this way the witness of an undiscovered form. It may possibly 

 belong to Peltocaris (p. 90). 



