336 THE LIAS AMMONITES. 



Description. — This is one of the Ammonites about which much confusion exists, arising 

 from the fact that in youth it very much resembles the young form of another Ammonite, 

 so that when of about the same age specimens of both species are frequently mistaken for 

 each other, whilst the morphological changes in form and structure with advancing life show 

 how widely dissimilar the two forms are before they reach their mature condition. The 

 one form, Aegoceras planicosta, Sow., appertains to the Lower Lias, and the other form, 

 Aegoceras capricornus, Schloth., belongs to the Middle Lias. 



The true position of Aegoceras planicosta was well defined by the slab containing 

 this species, which Sowerby figured in 1812. Here we see it associated with Arietites 

 obtusus in a mass of Marston stone found at Marston Magna, near Ilchester. Near 

 Yeovil, in Somerset, also, large slabs of a similar rock full of this Ammonite are raised with 

 their nacreous shells well preserved in clusters, some of the slabs are big enough to form 

 the tops of small-sized sideboards and other pieces of furniture. The beauty of the marble 

 depends upon the number of the Ammonites clustered together; the white pearly 

 layer of their shells is most admirably preserved, whilst the chambers are filled in 

 with fine brown crystallised carbonate of iron or carbonate of lime, which adds much 

 to the colour and richness of the dark grey ground of the Lias Marble. 



In the " Yorkshire Coast " the Rev. George Young early noted the specific character 

 of this Ammonite, in alluding to Bird's very bad figure (pi. xiii, fig. 6) ; he says, p. 259 

 (2nd edit., 1828), "It is from the lowest shale at Robin Hood's Bay [zone oi Arietites 

 obtusus'] ; it is a small and handsome Ammonite, similar to Ammonites maculatus, having ribs 

 that are sharp on the sides but flattened on the back. But the flat part in this Ammonite 

 bears a much greater proportion to the rest of the rib, and it is therefore named A. plani- 

 costatus, Sowerby, Tab. 73 and 406. It is generally found in a pyritous state. In the 

 older specimens a slight knob is sometimes found at each end of the flat part of the back." 



Professor Quenstedt also noted that the variety with the broad ribs on the back was 

 found only associated with Ammonites Turneri {=■ obtusus), from whence also the speci- 

 men figured in Zieten's " Wiirttemberg " came and not from Gammelshausen as stated, 

 where hardly any Lias is found. The figures given in the " Cephalopoden," Tab. iv, 

 fig. 6 a, b, are representations of one form of this Ammonite which I collected from the 

 jStellaris-hed at Lyme Regis, and which are drawn in PI. XXIV, figs. 1 and 2, of this 

 work, clearly proving the identity of the English and Wurtemberg shells. 



The Ammonites ziphus, Hehl, figured in Zieten, and which was determined by Mr. 

 Hehl, is distinguished principally by its elevated blunt spines placed at some distance 

 apart. This fragment was regarded by its author as a distinct species ; it was found in 

 the Lias Sandstone near Boll. When we compare Zieten's figures with the forms I have 

 figured from Lyme Regis, which are there associated with younger and older forms of 

 the same species, one cannot doubt their identity (PL XXIV, figs. 4 — 7). 



The next change in this species is characterised by the shortening of the ribs, which 

 terminate in small tubercles developed near the lateral boundary of the siphonal area, 



