﻿148 MK. S. S. BUCKMAN ON CERTAIN [Feb. I905, 



prove, and the more completely does it show the true genetic 

 affinities. The American palaeontologists have proved these state- 

 ments in regard to the Palaeozoic Brachiopoda. To use a generic 

 term in a wide sense, and then to group the species under their 

 respective Pormenreihe is hardly satisfactory : the formula is 

 much longer and more difficult to remember than a generic name. 

 Therefore, after noting some already-established generic names, 

 I venture to propose certain generic appellations for other genetic 

 series, with the species of which we are more particularly concerned 

 in our British Jurassic rocks. 



Genus Lytoceeas, Suess, 1865. 



Genotype, Ammonites fimbriatus, Sowerby. 



1865. E. Suess, ' Ueber Ammoniten ' Sitzungsberichte d. nat.-wiss. Classe d. k. 

 Akad. d. Wissensch. Wieii, vol. Hi, p. 78. 



Another species, Lytoceras lineatum, Wright (mow Schlotheim ?) , 

 ' Monogr. Lias Amm/ (Pal. Soc.) pi. lxix, fig. 1, 1882, & p. 409, 



1883. 



Genus Thtsanocekas, Hyatt, 1867. 



Genotype, Ammonites cornucopia, d'Orbigny (non Young) 

 = Thysanoceras Orbignyi, nom. nov. 



1867. Thysanoceras, Hyatt, Bull. Mus. Comp. Zool. vol. i, pp. 86, 96. 



Remarks. — Hyatt selected no genotype. The first species that 

 he mentions is Ammonites fimbriatus ; but this had just before 

 been taken by Suess as his type of Lytoceras. In Thysanoceras, 

 however, Hyatt placed many other Lytoceratoid species ; aud, as it 

 is now necessary to divide them into genera, it seems permissible to 

 take as the type of Thysanoceras a species most allied to Am. fim- 

 briatus. Therefore I select Am. cornucopia ; but, under this name, 

 Hyatt included the species of Young and of d'Orbigny. They are 

 two distinct species : as the latter is far the better illustrated, I 

 select that as the type of the genus ; it will require a new name. 



Thysanoceras, then, will be the name for the genetic series of 

 Am. cornucopia, d'Orbigny ; Lytoceras for that of Am. fimbriatus. 

 The two genera differ particularly in their shell -sculpture ; and also 

 that in Lytoceras the periodic ' flares ' are plain and prominent, 

 but in Thysanoceras they are crenulate and not prominent. 



These two genera attain to a high degree of development in 

 their suture-lines, 1 differing in that respect markedly from any of 

 the genera of the jurense-grouips (Pachylytoceras and like genera ; 

 see above, p. 144) : those still retain the phylloid saddles, indicative of 

 the original common ancestry of Lytoceratidae and Phylloceratidae ; 

 these have practically lost that ancestral trait; however, they 

 retain the primitive Auros-character. They do not seem to develop 

 smooth forms like the jurense-growps. 



1 Skeleton L, fig. 7, p. 152. 



