192 Evolution and Distribution of Fishes 



But a small mass of strata along the Moray Firth shore 

 known as the Linksfield Slates, and that is of doubtful 

 Wealden, Purbeck or Rhaetic age, deserves attention from 

 its fossils. These he lists (p. 148) as follows: 



* Femur of Trionyx sp. t Modiola hilliana 

 •Vertebrae of Plesiosaurus sp. f Modiola sp. 

 •Scales of Semionotus punctatus f/lstarte sp. 



* Scales of Lepidotus minor * Unio sp. 



* Scales of Pholidophorus sp. * Cyrena 



•Scales of Eugnathus sp. * Cyclas (several species) 



* Teeth of Hybodus laivsoni * Melanopsis sp. 



* Teeth of Hybodus dubius * Paludina sp. 

 t Teeth of Sphenonchits martini * Planorbis sp. 



* Acrodus sp. * Candona ? globosa 



* Spines of Hybodus *Estheria minuia var brodieana 

 t Ostrea sp. t Spine of Echinoderm 



f Pteroperna sp. * Neuropieris and other ferns 



f Myiilus sp. * Fragments of wood. 



Now as in the former case, so here, there is a sharply 

 marked set of lacustrine organisms, which we have indi- 

 cated by an *, as well as a set of marine remains indicated 

 thus f. And we venture to affirm that the two sets occur 

 in quite distinct, though it may be closely adjacent beds. 

 For all of the above fish remains had been up to this point, 

 and most of them remained, freshwater inhabitants. Acro- 

 dus and Hybodus alone were now wavering between a lake 

 and sea environment. 



Partial proof of the above is got on p. 162, where he 

 gives the exact succession and thickness of beds at Strath- 

 steven, and here steady alternation of marine and lacustrine 

 beds, with their appropriate fossils, is evident. But on 

 p. 182 an evident mixing up of the two sets of organisms 

 is set forth in his list. 



But in many lists of fossils from typically marine Juras- 

 sic strata, several genera of Cestracionts become increas- 

 ingly prominent alongside marine vertebrate organisms. 

 These are Hybodus^ Acrodus, Asteracanthus, and Stropho- 

 dus. The same is true, amongst chimaeroid selachians, of 

 the genera Myriacanthus, Squaloraja, Ischyodus, and 

 Ganodus. Unfortunately not a few of the species of these 

 genera are only known to us by their teeth, or by their teeth 



