FEOM THE DISTRICT OF ATHABASCA. 115 



Desmoceras affine, var. glabrum. 

 Plate IX. 



Placenticeras glabrum, "Whiteaves. 1889. Contr. to Canad. Palseont., vol. I., p. 172, pi. xxiv., 



figs. 1, la aud lb. 



Shell essentially similar to the typical form of the species in general shape and in 

 the ramifications of its suturai lines, but differing therefrom in the total absence of distant 

 arrests of growth in the septate portion. 



It is only proper to add that most of the specimens of this variety are abnormally com- 

 pressed, aud it is perhaps to this circumstance alone that the absence of the usual distant 

 periodic constrictions or arrests of growth is due. Pictet, in his description of the very 

 closely related Ammonites Bevdanli, ' says, in effect, in a passage formerly overlooked by 

 the writer, that the most compressed specimens do not show any of these arrests of 

 growth, and that they are always most marked in the most inflated individuals. He 

 has, he says, numerovis examples of A. Beudanti with these growth arrests (sillons), which 

 show that this character has no specific value. Some have only one constriction, some 

 two or three, and others more. They never have any when very young, and their earlier 

 volutions are perfectly smooth. All have perfectly similar suturai lines. 



The specimen upon which the description aud figures of Placenticeras glabrum were 

 based is a small but unusually perfect and well preserved cast of the interior of the shell, 

 collected by Mr. "W". Ogilvie in 1885 from the Loon Eiver Shales on the Peace River a few 

 miles below Fort Vermilion. It is ninety-six millimetres, or about three inches and three- 

 quarters, in its maximitm diameter, smooth and the whole of the ramifications of the lobes 

 and saddles of its crowded suturai lines are exquisitely shown. The suturai lines are so 

 complicated that it was not thought necessary to draw the whole of them, so that in this 

 respect the original figure is not an exact representation of the specimen, which was 

 referred to the genus Placenticeras on account of its supposed resemblance to the Ammo- 

 nites Clean and A. nisus of d'Orbigny, which Zittel places in that genus. 



Three larger but in every other respect precisely similar casts, the largest of which is 

 seven inches and three-quarters iu its maximum diameter, were collected by Mr. McCounell 

 in 1889, from the Loon River Shales on the Loon River, opposite the Buffalo Head Hills, 

 a locality about forty miles to the southward of that at which the type of the species 

 was obtained by Mr. Ogilvie. 



A few specimens with most of the test preserved were collected by Mr. McConnell in 

 1889, from the Peace River Sandstones on the Peace River at exposures twenty aud 

 twenty-five miles below Cadotte's River, also, from the Loon River Shales on the Loon 

 River thirty miles above its mouth. The largest of these specimens, which is represented, 

 three-fourths of the natural size, on Plate IX., is a little over nine inches in its maximum 

 diameter. The surface markings, as shown in this specimen, consist of numerous and 

 flexuous radiating strise, aud of several short aud very irregiilar faint spiral grooves or 

 strigations, but there is no evidence of any distant periodic arrests of growth. The outer 

 lip seems to have been considerably produced on the peripheral or ventral region. 



' Paléont. Suisse, Foss. Terr. Crét, env. Sainte-Crois, pp. 278-79. 



