GENEALOGY. 69 



Oxynoticeran Series. 



The apparently wide divergence from the usual structure of the Arietidae 

 presented by the hollow keel led me at first to classify this group as a distinct 

 family. The close affinities with the Arietidae shown by the young, however, 

 and the intermediate characters exhibited by the agassiceran series render such 

 a classification unnatural and undesirable. 



The loss of the keel and flattening of the abdomen in the old has no parallel 

 in the normal forms of the Arietidae, so far as known. 1 It must be remem- 

 bered, however, that this modification, together with its correlative decrease 

 in the lateral diameter of the whorl, is a fulfilment of the series of geratologous 

 transformations. 



First Subseries. — There are two subseries ; one, the oxynotum subseries, with 

 comparatively smooth shells, as in the less involute and stouter varieties of oxyno- 

 tum, and one, the Greenonghi subseries, which has highly developed folds. Both 

 of these series progress in the amount of involution. The less involute Oxyn. 

 oxynotum, the somewhat more involute Oxyn. Simpsoni? and the still more involute 

 Oxyn. Lymense of the first subseries are parallel with Oxyn. Oreenovghi, Guibali, and 

 Lotharingum of the second subseries. According to most authors, we could legiti- 

 mately consider that the first three were only varieties of one species, though 

 very few would be willing to join the last three under one specific name. The 

 sutures of the adults of the oxynotum subseries are very close together, reminding 

 one of the approximate sutures in the oldest stages of the individual in other 

 genera. The pointed lobes and broad saddles, the short abdominal lobe and 

 long finger-like marginal lobes and saddles, remind us also of the senile pecu- 

 liarities of the sutures of Cor. trigonatum. The increase in number of auxiliary 

 lobes and saddles and the general aspect of the sutures are, however, upon the 

 whole additional complications, and therefore progressive characteristics. 



Second Subseries. — The gradations are uninterrupted from Greenoughi 3 to 

 Lotharingum. The descriptions of the species show that the principal differences 

 consist in the increasing involution of successive species, and a correlatively 

 smoother and more compressed form of whorl. 



The young of oxynotum and Greenoughi resemble closely the young of Agas. 

 striaries. There are, however, no intermediate forms between the latter and 

 these two species which would enable one to verify this relation of the younger 

 stages. Whether Oxyn. oxynotum or Agas. striaries gave rise to Oxyn. Greenoughi 

 cannot be decided at present, owing to this deficiency in the evidence ; but that 

 both these forms came from the same common stock, and that this stock was 

 Agas. striaries, seem quite probable. 



Oxyn. Guibali, the next species of this subseries, bridges the gap between 

 Greenoughi and Lotharingum. The nealogic characters of Oxyn. Lotharingum also 

 show that this species must have been derived from Guibali. Lotharingum, like 

 Ast. Collenoti and other terminal forms of series which have marked geratologous 

 characters, is the smallest form of its own genetic line. 



1 PI. x. figs. 24 and 31. 2 Summ. PI. xiii. fi ? . 11. » Sumra. PI. xiii. fig. 13. 



