644 w - D - SMITH 



ability of their being in close relationships with the ammonoids that 

 Logan cites. 



At o.66 mm diameter a constriction was found on one individual 

 (Number 12, Fig. 1), while again specimens were broken back to the 

 protoconch and not a constriction seen. In others it was found 

 recurring several times in the same individual. However, there is 

 no constancy about this feature, as in Baculites, Desmoceras, and 

 many others (Number 2, Fig. 2). In Baculites this is, according to 

 J. P. Smith, a very constant feature, and comes always at the end 

 of the larval coil, i. e., one revolution. In several specimens of 

 Scaphites condoni and S. inermis this constriction has been noted by 

 the writer at about three-quarters of a revolution from the proto- 

 conch. 



In the Montana species this is taken by the wrier to be purelyt 

 an individual character, and one likely to be found at almost any 

 point in the development of a phylogerontic group. It means simply 

 that there is a pause now and then in the growth of the animal, as 

 should be expected in the members of a group that we know was 

 then on the decline. 



From this point on the shape of the aperture changes rapidly from 

 a crescentic to a more roundish form, due to heightening of the 

 walls, and when the adolescent period is reached, the aperture is 

 almost circular (Number 16, Fig. 1). The adolescent period, in the 

 species studied, has not been found to begin at any constant size. 

 The notching of the first lateral lobe, when it becomes, to use a 

 techincal term, bifid, is assumed to be the beginning of the adolescent 

 period. In one specimen this stage began at a diameter of i.87 mm , 

 in another at 2 mm , in a third at 2.26 mm , while a fourth continues in 

 a larval state up to 2.48 mm , or 2^4 revolutions. 



Number 14, Fig. 1, shows a smooth shell of this form and the sutures. 

 The umbilicus is quite wide still, and the shape of the aperture little 

 changed. From this point on, however, the whorl increases rapidly 

 in height and width, and the whole shell becomes thicker. In S. 

 nodosus var. plenus this growth is very sudden and marked, as will 

 be seen by referring to 7 and 8, Fig. 3. 



Some measurements to show the relative growth at the different 

 sizes will doubtless be more useful than a verbal description: 



