COLWICK WOOD, NEATl NOTTINGHAM. 



543 



one another in the middle portion of that distance, but gradually 

 becoming more widely separated in either direction until they 

 finally came to an end ; and, secondly, their occurrence at the 

 junction of two subformations of the Trias, namely, of the Water- 

 stones of the Upper Keuper and the Basement-beds (Lower Keuper). 

 This may be, and probably is, merely an accidental coincidence. 

 At the same time, it may be worth while to record the fact. The 

 two series of deposits, at the junction of which this fossil shoal of 

 fishes was found, are of very diverse characters, and were formed 

 under very different physical conditions. The Keuper Basement- 

 beds are a series of gritty, false-bedded sandstones with frac- 

 tured quartzite pebbles and strange wedge-shaped intercalations of 

 fine red marl and marly debris, irregularly bedded and showing 

 clear signs of the existence of powerful currents as well as of 

 considerable contemporaneous erosion. These deposits I believe to 

 have probably had a fluviatile origin. The Waterstones, on the 

 other hand (at the base of which the fishes occurred, and to which 

 series they belong), are regularly bedded fine-grained sandstones ,and 

 marls, showing ripple-marks and sun-cracks*, and were evidently 

 formed in waters which were tranquil but extremely shallow, and 

 liable to entire and perhaps rapid desiccation. These waters were 

 in all probability those of saline lakes or lagoons. Possibly the 

 fishes found at Colwick may have become entrapped in the shallows 

 of such a lake, and killed in numbers by the drying-up or the 

 increasing salinity of the water. 



EXPLANATION OF PLATE XXII. 



Fig. 1. Semionotus, found by the Rev. P. B. Brodie in the Upper Keuper of 

 Shrewley, twice natural size. d, dorsal fin ; p, pectoral fin ; 

 v, ventral fin ; an, anal fin. The head and greater part of the tail 

 are wanting, but their probable form is indicated by a hypothetical 

 outline. 



Fig. 2. A much crushed specimen, natural size, showing the large fulcral 

 scales of the tail. 



Fig. 3. Another example, twice natural size, showing part of head, pectoral 

 fin, and possibly part of ventral fin. 



Fig. 4. A fragment, showing ventral fin, twice natural size. 



Fig. 5. A tail, twice natural size, showing its moderately heterocercal cha- 

 racter. 



Fig. 6. A similar specimen, also twice the natural size, showing the large 



fulcral scales. 

 Fig. 6 a. A few of the scales, further enlarged. 



Fig. 7. Scales, enlarged, from a specimen presented to the British Museum 



by the Rev. P. B. Brodie, showing their denticulate margins. 

 Fig. 8. One of the numerous specimens of Semionotus found by Mr. E. Wilson 



in the Trias of Colwick Wood, near Nottingham ; natural size. 



Other examples show that the ventral fins (and probably the anal 



fin) were situated as in figure 1 above. 

 Fig. 9. A scale, enlarged 20 diameters, of one of the Palasoniscoid fishes, also 

 " found by Mr. E. Wilson in the Trias of Colwick Wood. 



* In these lowest beds of the Waterstones at Colwick I found the stem of a 

 land plant having the appearance of Eqaisetites columnaris, and probably 

 allied thereto. Unfortunately it was only a sandstone cast, and too friable to 

 remove. 



2o 2 



