﻿Vol. 6 1.] CEKTAIN GE3S T ERA AXD SPECIES OF LYTOCEKATIDJi;. 153 



seen to have a similar pattern — that is, a lobe of later phylogenetic 

 origin in a more highly-developed lobe-line is similar to a lobe (L) 

 of earlier origin in a less-developed line. While the L of Mega- 

 lytoceras has passed far beyond the L of the jurense-grou^s, the I 

 (the lobe of later origin) of the first has come up to about 

 the same stage as the L of the second. 



This observation somewhat confirms those of E. T. Jackson, 1 

 with a difference. He noted that a law of progressive and successive 

 development, a kind of ontogenetic repetition of phylogeny, applied 

 to the elaboration of any parts of an individual as much as to the 

 whole ; and he adduced as instances the individual lobes of the same 

 specimen during ontogeny, and the various lobes and lobules of the 

 same suture-line. In the present case, there appears to be a similar 

 law of development in the lobes of different stocks considered as 

 parts of a family, the stocks having elaborated their suture-lines to 

 different degrees of development. In the case of two stocks, one of 

 which has carried the development of its suture-line much farther 

 than the other, the less-developed lobe of the most-developed stock 

 is found to be similar to the most-developed lobe of the less- 

 developed one. 



III. Summary. 

 New Genera. 

 Pachy lytoceras, Genotype Ammonites torulosus, Zieten. 

 Thysanolytoceras, „ Am. Eudesianus, d'Orb. 



Megalytoceras, „ Lytoceras confusum, S. Buckm. 



Nannolytoceras, ,, Ammonites pygmams, d'Orb. 



New Species. 

 Pachy lytoceras aalenianum, Aalenian. 



„ phyllcceratoides, ,, 



New Names. 

 Alocoly toe eras Pompeckji for Ammonites Oermaini, d'Orb. 



pi. ci, figs. 1 & 2. 

 Thysanoceras Orbignyi for Am. cornucopia, d'Orb. (non 



Young), pi. xcix, figs. 1-3. 

 Thysanoceras onychograptum for Am. cornucopia, Wright 



(non Young). 

 Generic name revised. 

 Thysanoceras, Hyatt. Type selected, Am. cornucopia, d'Orb. 



= Th. Orbiynyi, nov. 

 Species figured. 

 Alocoly tocer as tceniatum (Pompeckj). 

 Pachy lytoceras aalenianum, nov. 



„ phylloceratoides, nov. 



Megalytoceras confusum (S. Buckm.), lobe-line of. 



1 'Localized Stages in Development in Plants & Animals' Mem. Boston 

 Soc. Nat. Hist. vol. v, No. 4 (1899) pp. 134 ci seqq. 



