438 THE LIAS AMMONITES. 



species, he noticed that, as in other carinated Ammonites, the keel did not appear until the 

 shell had attained the size of from two to four millimetres in diameter, and commenced to 

 develop at the same time, the folds on the sides of the whorl indicating the appearance of 

 the ribs. The deep lateral sulcus made its appearance when the shell had attained seven 

 millimetres in diameter. The then shell continued to grow with all its ornaments. The 

 whorls were proportionally larger in the young than in adult specimens. Soon the ribs 

 became less prominent, and the sides less inflated, and when the diameter of 140 

 millimetres had been reached the ribs entirely disappeared, and the shell appeared almost 

 smooth, and the lateral sulcus was then only feebly marked and less shallow than in 

 early life. 



Affinities and Differences. — Harpoceras bifrons makes one of the remarkable group 

 Falciferi, among which there are many characters common to the whole. It resembles 

 very much Harpoceras serpentinum, and is regarded by some as only a variety of that 

 species ; there are, however, several organic features which distinguish them : 1. The 

 three carinse on the siphonal area and the two lateral sulci on each side of the median 

 keel ; 2. The deep longitudinal sulcus which divides the whorls along their length, the 

 outer two thirds of the side having convex ribs, and other narrow portions being inflated 

 and smooth ; 3. The suture-line is very different also in the two forms. It has many 

 affinities with Harpoceras Levisoni ; these, however, will be pointed out in the following 

 article, which will embrace the history of that species. 



Locality and Stratigraphical Position. — This is one of the characteristic fossils of the 

 Upper Lias, and is the type form of the zone it represents ; it is found at Crickley, 

 Frocester, and Stinchcombe Hills sections, and at Alderton, Grettan, Dumbleton, Glou- 

 cestershire ; at Bloxam, Oxfordshire ; several localities in Rutland, Santon, north-west 

 Lincoln ; and Whitby, Yorkshire, the metropolis of the species ; from the last locality 

 my beautiful figured specimen was obtained. 



Foreign Distribution. — I must refer to my ' Correlation of the Harp. bifrons-zoTie,' 

 pp. 3^27 — 136 of this Monograph, for an exhaustive account of the distribution of this 

 horizon of life in north and south Germany, France, and Switzerland. 



Harpoceras Levisoni, Simpson. PI. LX figs. 1 and 2 ; and LXI, figs. 1 — 6, 



Ammonites Levisoni, Simpson. Monogr. on Ammonites of Yorksh., p. 99, 1855. 



— BOKEALis, Seebach. Der Hannover. Jura, p. 140, taf. vii, fig. 5, 1864. 



— coMENSis, Beyn^s. G^ol. et. Pale'ontol. Aveyron, pi. 5, fig. 6, 1868. 



— Levisoni, Bumortier. Dep6ts Jurass.,part. iv, p. 49, pi. ix, figs. 3, 4, 1874. 

 Harpoceeas — Tate and Blake. Yorksh. Lias, p. 307, 1876. 



Diagnosis.— ^\ie\\ discoidal, compressed, carinated, with a very wide umbilicus; 



