AEGOCERAS CAPRICORNUS. 369 



and states that the name Ammonites capricornus was given by Schlotheim to a species, 

 examples of which are there displayed from Whitby (England), and from Amberg and 

 Heinberge near Gottingen (Germany). All the specimens which he saw with the ticket 

 Ammonites capricornus undoubtedly belonged to the species which Professor Quenstedt in 

 his works calls Ammonites maculatus. 



Diagnosis. — Shell flattened and discoidal, whorls round and 

 slightly involute ; ribs annular, sharp, and bent towards aperture, 

 with wide concave spaces between ; siphonal area round, rib-arch 

 prominent, with numerous striae between ; aperture circular ; 

 umbilicus wide. 



Dimensions. — Transverse diameter 80 millimetres ; width of 

 the umbilicus 45 millimetres ; height of aperture 25 millimetres ; 

 transverse diameter 22 millimetres. 



Description. — This is a variable Ammonite as regards the 

 form and size of the ribs, which are quite annular, thickened, comus, Schioth. 



and flattened where they pass across the siphonal area. After 



selecting from a number of specimens I consider the shell figured PI. XXXIV, figs. 1, 

 2, 3, a good typical example, and in figs. 4, 5, 6 of the same, I have had a larger 

 specimen delineated. Here we observe the outer whorl becomes greatly enlarged, and 

 two rows of tubercles make their appearance with the commencement of the sixth whorl, 

 whilst the annular ribs of its former condition have assumed a new livery. This is very 

 well seen in the side view where the tubercles first appear, and the great enlargement of 

 the last whorl is well expressed in fig. 6, where the ribs proceeding from the tubercles 

 are seen to split, and the intermediate spaces are covered with transverse striae. 



The figures in this plate are arranged to show that Aeg. capricornus or maculatum 

 as we usually meet with it, is the young form of a shell resembling Am. liyhrida of 

 d'Orbigny, figs. 4, 5, 6, and which in better developed specimens we shall find to be 

 the true Am. heterogenes of Young. 



Affinities and Differences. — The young form of this Ammonite resembles the specimen 

 figured by d'Orbigny as Am. planicosta, which certainly is not the planicosta of Sowerby. 

 I have already shown by the figures and description of Aeg. capricornus that it 

 resembles, however, Am. latacosta, Sow., whilst the round whorls, straight annular ribs 

 without tubercles and circular mouth aperture, indicate close affinities with a nearly 

 allied species. 



Stratigraphical Position. — This is a very characteristic Ammonite of the zone in 

 which it is found at the base of the Amaltheus margaritatus beds at HuntclifF, Robin 

 Hood's Bay, Staithes, Coatham Scars, and Guisborough, Yorkshire Coast. 



48 



