PHYLLOCERAS HETEROPHYLLUM. 427 



auxiliary lobes all preserve the same form, but become less and less complicated as we 

 trace them from without inwards. When we describe a radial line from the digital 

 points of the siphonal lobe to the digital points of the auxiliaries, it touches the points 

 of all the others, and cuts the terminal branches of the principal lateral. The near 

 approximation of the chambers, and the great development of the suture-line, produces on 

 the surface of the mould of this Ammonite a number of figures which are as difficult to 

 trace out as they are beautiful to behold. 



Professor d'Orbigny had an opportunity of examining several young forms of this 

 species, and stated that a shell ten millimetres in diameter had absolutely the same form 

 and the same lobes as the adult, and was entirely smooth, and that it did not acquire 

 stri« until it had attained a diameter of from twenty to thirty millimetres. 



The mould is always entirely smooth. When the inner lamina of the shell was preserved 

 he remarked that striae were only slightly visible, but when the external layer of the shell 

 existed the striae were well shown.^ 



The late Mons. Dumortier^ observed of this Ammonite " that the veritable A. hetero- 

 pliylliis. Sow., is very difierent to the A. Zetes of the Middle Lias. The specimens of 

 large size from Verpilliere, Isere, are remarkably beautiful, both as regards their well- 

 preserved forms as well by the perfection of the details of their ornaments ; " and again, 

 " upon some examples of moderate size we remark from six to seven radiated depressions, 

 very slight upon the last whorl, but which terminate upon the siphonal contour by a 

 round prominence and form a sinus in front. As all the other characters remain unaltered 

 this peculiarity is to be considered as an accidental variety of A. heterophi/Uus." This was 

 doubtless a mould of the fan-like portions of the body-chamber which had lost its shell, 

 whilst the fragments which Duraortier figured as " cor^JS de nature inconime " were in fact 

 the shell of the fan without the mould on which it rested. 



Affinities and Bifferences. — In the Jurassic rocks we find five species of Ammonites, 

 which may all be referred to the group Heterophyllida . 1st. Phyll. Loscombi of the 

 Middle Lias, with a large umbilicus, without ribs in early life, and the suture forming 

 six lobes. 2nd. PJiyll. Zetes, d'Orbigny, from the Middle Lias (the Amal. spinatus-zone) 

 distinguished from P/iyll. heterophylliim by more compressed whorls, a wider umbilicus, and 

 a difierent suture-line. 3rd. Fhyll. heterojjJiylhm, the type of the group, which has just 

 been described in detail. 4th. Pliyll. Calypso, from the Upper Lias, distinguished by the 

 five flexed contractions observed on the last whorl of the spire. 5th. The PhylL tatricum, 

 Pasch, from the Lower Oxfordian, with six to eight bent constrictions of the outer whorl, 

 which is very involute and covered with a thick shell finely sculptured, with minute striae. 

 All these Jurassic Heterophyllidce are very distinct from other members of the same natural 

 family which are found in the Cretaceous formations. 



Locality and Stratiyraphical Position, — Phylloceras heterophyllum, Sow., is found in 



1 ' Paleontologie Franfaise, Terr. Jurassiques,' torn, i, p. 340, 1842. 



2 ' Depots Jurassiques du Bassin du Rhone,' torn, iv, p. 105, 1874. 



