62 GENESIS OF THE AKIETID.E. 



of the pilte and of the channels, but often left the keel more prominent. The 

 whorl acquired the true flat-sided trigonal form, but never became rounded, so 

 far as jet observed. 



LEVIS STOCK. 

 Arnioceran Series. 



This series begins, when zoologically considered, with Am. miserable, 1 a form 

 very commonly in collections named Amm. planorbis or psihtwtus, on account of its 

 close external resemblance to that species. 2 It has. however, distinct sutures, and 

 acquires by growth a subacute abdomen. During the younger stages, while it is 

 still round on the abdomen, or in varieties with broad abdomens, it is. with the 

 exception of its smaller size, a close reproduction of the adult of Psil planorbe, 

 var. kre. This grades into Am. miserabik, var. cunetforme, 3 which has a more 

 acute abdomen and curved and more perfect pilae acquired at an earlier age, 

 and from this without a break the series passes into Am. obtusiforme.* 



Starting again from variety acutidorsale of miserabik, we can follow another 

 line of affinities. There are some forms of this variety which acquire in the 

 adult a keel with faint but abruptly terminating folds or pilae, and these lead into 

 a variety of Am. semicostatum* This species has many varieties, which grade 

 from an immature planorbis-like form 6 to those which are prominently keeled 

 and pilated even at a comparatively early age, 7 and also into varieties which have 

 deep channels on the abdomen. 8 These last are inseparable from Am. tardecres- 

 ccm. 9 and when they have numerous pilse they are inseparable from Am. ceras? 

 There are also varieties of Am. semicostatum which fade into Hartmanni, 11 and 

 this in turn grades into the still more compressed Bodte/ji} 2 From Am. Hart- 

 manni, also, we can pass into another compressed form, the true Amm. falcaries 

 of Quenstedt. 13 



Returning again to semicostatum, we find that one "of its varieties is distin- 

 guished for remarkably raricostatus-like pilaa and a low keel. This when fol- 

 lowed out leads to Am. kridioides. u This species in some of its varieties so closely 

 resembles Cal. raricostatum, that for a time it was thought to indicate the direct 

 descent of that species from semicostatum. 



There are also some forms, usually identified as Amm. Tcridion in Germany, 

 which have remarkably broad whorls in the adult, and approximate to the true 

 kridion. These, however, never possess the tubercles of the true kridion, and 

 also have young which prolong the smooth stage and otherwise resemble the 

 young of the stouter forms of semicostatum. 



I PI. ii. fig. 4, 5; Summ. PI. xii. fig. 2. 



- The apertures arc also similar as figured by Quenstedt, Amm. Schwab. Jura, pi. xiii. fig. 27, by Du- 

 niortier. 



PI. ii. fig. 7. ■" PI. ii. fig. 8, 0; Summ. PI. xii. fig. 3. 



5 PI. ii. tig. 10; Summ. PI. xii. fig. 14. « PI. ii. fig. 10. 



7 PI. ii. tig. 11. * PI. ii. fig. 15. 



9 PI. ii. fig. 19; Sum. ii. PL xii. fig. 6. I0 PI. ii. fig. 20, 20 a. 



II PI. ii. tig. 17: Summ. PI. xii. fig. 5. 12 PI. ii. fig. 2:3; Summ. PI. xii. fig. 7. 

 » PI. ii. fig. 2.3 27. " PI. ii. fig. 28; Sumin. PI. xii. fig. S. 



