142 NATURAL SCIENCE. Aug., 



marginal curve in fig. 4 is greater than in fig. 5, and that in fig. 3 

 greater than in fig. 4. We should have supposed that to estimate the 

 curves of these shells it was necessary to take the mean of the 

 peripheral and anti-peripheral curves — by which method, of course, 

 fig. 3 is curved the most. 



Professor Blake takes all this trouble about the curves to prove, 

 first, that " an involute Ammonite (Phylloceras) has move curvature than 

 an open -whorled Crioceras " ; secondly, that " the difference between 

 the form of Phylloceras, Arietites and Crioceras has nothing xvhatever to do 

 with their curvature," — two statements which do Professor Blake 

 great credit, as they do not contradict each other in the least ! 



In dealing with the involution, Professor Blake makes the 

 remarkable statement that Ammonites are necessarily more evolute 

 in youth than in later life. Am. salisburgensis, Hauer ; Amm. gonionotum 

 and fallax, Benecke ; Amm. polymovphus and dimorphus, d'Orbigny ; 

 Am. polymcrus, Waagen ; Am. brocchi, Sowerby ; and the majority of 

 the Stcphanocerata support this view, we suppose. Really Professor 

 Blake's perception must be keen ; he sees what no one else could do 

 in the matter of these Ammonites. Then he proceeds to say that 

 Hyatt and Buckman seek an evolute ancestor for every form, and 

 finds fault with them in consequence. How confidently the members 

 of the Geologists' Association may rely on Professor Blake's accuracy ! 

 Hyatt (" Genesis of the Arietidse ") gives an involute form like Icevigatus 

 as the ancestor of certain species ; Buckman (" Monograph," p. 283) 

 places an involute shell like globosus as the ancestor of the Poly- 

 morphidae, and also (p. 289) states that the first stage of Ammonite 

 ontogeny discloses a form like globosus, such, therefore, is the ancestral 

 form he would seek. 



In connection with the thickness (p. 28), Professor Blake 

 announces a discovery which is new to us : " the whorls of an 

 Ammonite become naturally more compressed with growth." What 

 a number of unnatural Ammonites there must be ! For instance : 

 Amm. bechii, Sowerby; spinatus, Brug. ; sauzeanus, d'Orbigny; semi- 

 costatus (Young), Hyatt ; adela, d'Orbigny ; francisci (Oppel), Vacek ; 

 Sutherland^, d'Orbigny ; goliathus, d'Orbigny ; not to mention many 

 others which change from less compressed to more compressed whorls, 

 and again to less compressed in infancy or adolescence, and change 

 again to more compressed in adolescence or maturity. We should 

 have thought that the only law which could be laid down safely was 

 that, in anagenesis, the whorls tend to become more than propor- 

 tionally inflated, and, therefore, less compression is likely to arise ; 

 while in catagenesis the whorls tend to be less inflated, and greater 

 compression may result. Unfortunately, the law is not so simple 

 as this. Catagenesis may produce a less compressed whorl, because 

 the reduction, in proportionate capacity, may be attained by reducing 

 the breadth and not the thickness of the whorl. 



The size of the Ammonite is given as one of the bases of classi- 



