326 Reviews — Prof. A. Hyatt — Genesis of the Arietidce. 



Jurassic Ammonites, as well as of Cephalopoda generally ; and the 

 important and valuable monograph now before us treats not only of 

 the genesis, but also of the entire specific evolution of the grand 

 Liassic Ammonite family, the Arietidaa. Not the least important 

 feature of the work is that it is illustrated by fourteen excellent 

 plates prepared expressly to show the developmental changes and 

 the genealogy of the various species. 



The principles of Ammonite development, which, we believe, owe 

 to Prof. Hyatt their first enunciation and application, though they 

 have also been put forward by other workers independently, are, in 

 the present work, fully discussed in two long chapters. " Genealogy " 

 and the " Genesis of Characteristics " are treated in two other 

 chapters ; on " Geological and Faunal Relations " is the title of a 

 chapter discussing the birthplaces and migrations of the different 

 series ; and then lastly follows a chapter describing the genera and 

 species. 



With the main results of Prof. Hyatt's labours we find ourselves 

 in complete accord ; but we might be inclined to criticize some of 

 his genealogic conclusions. We are, however, disposed to be cautious 

 in this matter, as we have not the number of specimens to appeal to. 



Still it seems to us that to trace the Arietidas to Psiloceras planorbe 

 may be a mistake. This species we regard — on account of the ribs 

 in the inner whorls and its slightly more involute form — as a smooth 

 development of Am. Johnstoni, and not the ancestor thereof. That 

 Am. Johnstoni and the Arietidse were derived from a very evolute, 

 perfectly smooth form, and that this was derived from an involute, 

 gibbous-whorled, smooth form — a shell more evolute than Nanniies 

 fugax and more involute than Agassiceras Icevigatum, something like 

 N. fug ax being the ancestor producing forms similar to Ag. lavigatum 

 and Am. miserabile as it became more and more evolute — seems to be 

 indicated by the inner whorls of species which we have broken up. 



However, space does not allow us to discuss this and certain other 

 points ; but we wish to say that this work has given us extreme 

 pleasure, and that we cordially congratulate the talented author upon 

 the splendid results which this monograph lays before us — results 

 which indicate a vast amount of careful and critical research. Not 

 only is a study of this monogi'aph absolutely essential to the student 

 of Ammonites, or of Cephalopoda ; but it is quite as necessary to the 

 student of Lamellibranchiata, Gasteropoda, etc., because the same 

 principles of development hold good. 



One little matter we feel bound to mention. We dislike to see well- 

 known species like Am. planorbis, Sowerby, written as " Psiloceras 

 planorbe, Hyatt." This method was condemned by a committee of 

 celebrated naturalists in 1842, ' and the practice of European authors 

 who write " Psiloceras planorbe (Sowerby)," or " Sowerby sp." 

 sufficiently supports their decision. S. S. B. 



1 On Zoological Nomenclature, Report British Assoc, vol. xi. See also Rapport 

 du. Commission du Congres international, Paris, 1881, p. 4. 



