204 A. HYATT ON THE PARALLELISM BETWEEN THE INDIVIDUAL 



in order such species as Amm. Ceras and Amm. Comjbeari, which are non-involute, like the 

 Clymeniae or lower Nautili, and have smooth ribs with a keeled and channelled abdomen ; 

 then Amm. Bucklandi, Amm. bisulcatus, Amm. Brookei Sow., which differ from the first in their 

 tuberculated ribs ; and finally Amm. Brookei Zieten, Amm. obtusus, Amm. stellaris, and Amm. 

 Collenotii, devoid of tubercles and with the ribs more or less depressed near the abdomen. 

 This part, also, becomes narrower than in the preceding species, and in the adult of the last 

 three its channels are lost and the keel is almost obsolete. The deterioration is so exten- 

 sive and perfect, that Amm. Collenotii has been deemed by some authorities one of the Clypei- 

 formi with a smooth, acute abdomen. Both Amm. stellaris and Amm. Collenotii are more invo- 

 lute than the others, but they have similar septa, and it is difficult to draw the line between 

 the former and some varieties of Amm. obtusus, an undoubted Arietian ; the thick, depressed 

 ribs of Amm. stellaris are traceable, also, upon the sides of Amm. Collenotii, although, of course, 

 one degree nearer to absolute atrophy. 



In the old of Amm. Bucklandi or Amm. Brookei Sow., the prominently keeled, flat, and 

 deeply channelled abdomen, and the projecting, tuberculated ribs of the adult may be ob- 

 served giving way to a narrower abdomen, with a depressed keel and without channels, 

 the ribs obsolete near the abdomen and the aspect of the whorl in transverse section 

 changing from the rectangularity of the adult to a trigonal outline. The increase of the 

 radii of the spiral and the continued complication of the septa are not interfered with, 

 and the shells show no perceptible signs of senility. 



These old age peculiarities are identical with those distinguishing the adult of Amm. 

 Brookei Ziet., (which is a totally different species from Amm. Brookei Sow., separable by rea- 

 son of the trigonal outline of the whorl and the depressed, smooth ribs of the adult) and 

 Amm. tenuis nob. MSS. 1 The latter, however, is deeply channelled in the adult, while the 

 former is so only at an earlier stage, but they both repeat in the young the rectangular 

 whorl of the preceding species. Amm. stellaris has the keel and channels more distinct 

 in the young than in the adult, but the form of the whorl shifts at an early period to the 

 senile trigonal aspect and becomes quite involute in the adult. The last is the only char- 

 acteristic which is not identical with the old of Amm. Brookei Ziet. In Amm. Collenotii the 

 rectangular whorl is entirely left out of the growth, the transition being gradual from the 

 round embryonic to the senile trigonal, and the channels are only faintly marked in the 

 young. The whole shell is an exaggeration of the old age tendencies of Amm. bisulcatus or 

 Amm. Brookei Zieten, and the adult is a counterpart of the extreme old age of the involute 

 Amm. stellaris, when the ribs are almost wholly obsolete and the sides are partially smooth. 

 Amm. stellaris and Amm. Collenotii, therefore, are the old age forms of the Arietes series, and 

 they, also, complete the senile tendency shown in the old individual of the lower forms by 

 becoming more involute, — that is, by increasing the radii of the spiral in greater propor- 

 tion than any other species, and thus responding, as the heads of the series, to the unceas- 

 ing increase of the radii in the spirals of Amm. Bucklandi and the like. 



Following the keeled group upwards, the " homomorphs " of the old age forms of the 

 Arietes are found to acquire greater importance, until in the Cretaceous they are repre- 

 sented by an entire series of smooth, involute shells with acute abdomens and no channels. 



In the Lower Lias all of the Arietes, with the exception of the two senile forms, are non- 

 involute. The Amalthei of the Middle Lias, however, have but one species, Amm. Hawsker- 



l Also to be published in the Museum Bulletin. 



