AND ORDER IN TETRABRANCHIATE CEPMALOPODS. 205 



ensis with an open umbilicus, the rest, including Amm. costatus, vittatus, and margaritatus, are 

 progressively more involute with more complicated septa, and their abdomen more acute. 

 Succeeding these are the Falciferi of the Upper Lias, which have but two species, Amm. 

 Walcotii and Amm. bifrons, that simulate Amm. Hawskerensis and the Arietes, and then a long 

 list of others, first with acute abdomens and open umbilici, as in Amm. striatums, then more 

 involute as in Amm. radians and serpentinus, and finally with the internal whorls entirely 

 hidden, as in Amm. discoideus and Amm. elegans. The remaining strata of the Jura contain 

 but two or three, like Amm. cijcloides D'Orb., and some of the so-called varieties of Amm. 

 MurcMsonice, which have the abdomen channelled and keeled ; their enrolment in all cases, 

 however, that have come under my eye, is more involute than the generality of the 

 Arietes. The Cristati and Clypeiformi constitute the Cretaceous series. The first has a few 

 species, such as Amm. injiatus and Amm, tricarinatus, which repeat the narrow-whorled Arietes, 

 but only one species, Amm. tricarinatus, has the umbilicus sufficiently open to emulate with 

 any accuracy the non-involution of Amm. Walcotii of the Falciferi, or Amm. Conybeari of 

 the Arietes. The rest are all more involute, and have, in proportion to the radial breadth 

 of the whorls, narrower abdomens. The second series is wholly composed of shells which 

 imitate the old age involution, the smoothness, the obsolescing keel, the obsolete channels, 

 the acute abdomen and trigonal whorl of Amm. stellaris and Amm. ColknoiU. 



Besides these, other instances might be taken demonstrating the incorporation of the 

 degradational characteristics of the individual into the higher forms of succeeding groups, 

 but the brevity of my remarks does not admit such a full illustration of the subject. 



One more example is needed, however, to establish the fact, that this incorporation of 

 old age tendencies in the growth of succeeding shells becomes more complete not only 

 as we approach the upper boundaries of the life of the order, but, also, in proportion to our 

 proximity to the termination of the structural cycle among the polar forms. 



The senile characteristics of the Clypeiformi are brought out in a manner closely analo- 

 gous with those of the smooth Nautili, the progress of the septa and form of the whorl in 

 complicating themselves is not sufficiently impaired to afford well-marked degradations, 

 but in the series of shells with round abdomens, like Amm. Ilumphriesianus, a different ele- 

 ment is introduced. With them the entire organization appears to have finished its ad- 

 vance and to have begun its morphological decline; the adult era of the collective life of 

 the order is passed, and the coming of its old age era fairly announced. The old of Amm. 

 Ilumphriesianus of the Inferior Oolite, as noticed by D'Orbigny, 1 varies the rate of increase 

 in the length of the radii of the spiral, and ceases to augment the volume of the whorl, 

 which is less involute and more tubular than in the adult. This is attended, as we have 

 seen, by a narrowing of the cells consequent upon the lessening of the space which the 

 lobes have for their expansion. The diminution of the radii in the old of Amm. dimorphus 

 and Amm. Sauzei, also, of the Inferior Oolite, is more abrupt, and if persisted in would pro- 

 ject the last portion of the whorl after the fashion of a Scaphites, or Crioceras. Ascending 

 to the Great Oolite, we find two singular species of the same series, Amm. microstoma and 

 Amm. bullatus, which are eccentric from an early period. The chambers of Amm. microstoma, 

 as they are successively built and left behind in course of growth, become more tubular, 

 and describe curves within the limits of the ideal prolongation of the regular spiral. 

 These chambers are not reabsorbed but remain undisturbed, the animal building onward 



l D'Orbigny, Reclierches sur lis A mmoniles. 



