366 THE LIAS AMMONITES. 



Description. — The above diagnosis and the following description are in Sowerby's own 

 words :— " The flattened sides of this Ammonite distinguish it from the young state of 

 ^.jofemcos^a independently of its much larger size. When young neither has any appear- 

 ance of spines ; when old the last whorl of the latacosta has only slight indications of 

 tubercles, which consist of two small knots upon each ray on each side. The planicosta 

 has one large spine in place of two, and that only upon some of the rays. A Lias fossil. 

 All the specimens I have seen except one are from the alluvium ; that one is from Lyme ; 

 it is about three inches and a half in diameter, and was liberally presented by Mrs. 

 Murchison. One specimen, nearly four inches in diameter, shows the little knots upon 

 the rays ; it is in the collection of W. Peete, Esq., of Dartford. Several small ones have 

 been found by Miss Baker, of Braunston, in what is called a gravel-pit." 



Having examined the original Sowerbyan type of latcecosta, and compared it with other 

 Ammonites from the same Green Ammonite-bed, I am prepared to state that latcecosta, 

 Sow., is the middle-age condition of Aegoceras Henleyi, which I shall describe in the 

 following article. As this statement may excite doubts in the minds of some persons, 

 I have taken the precaution to have Sowerby's type shell, now in the British Museum, 

 drawn, PI. XXXII, fig. 1 ; and I have quoted in extenso Sowerby's own words, viz. 

 those he had written on the subject ; for nothing is so difficult to eradicate as our early 

 notions, whether true or false, of the specific forms of organic bodies. 



Aegoceeas Henleyi, Sowerhy. PL XXXIII, figs. 1, 2, and 3. 



Ammonites Henleyi, Sowerby. Mineral Conchology, vol. ii, p. 161, tab. 172, 



1817. 

 — Henleyi, Seynes. G^ol. et Paleontol. Aveyronnaises, p. 88, pi. i, fig. 2, 

 1868. 

 Aegoceras — Tate and Blake (i^&rs). Yorkshire Lias, p. 281, 1876. 



Diagnosis. — Shell with external whorl inflated, internal whorls discoidal and com- 

 pressed ; composed of six volutions, all exposed ; inner whorls with simple, obtuse, 

 annular ribs, separated by concave valleys, the ribs flattened as they pass over the 

 siphonal area ; part of the outer whorl much expanded, and ornamented with numerous 

 fine, narrow ribs on the sides, which split up beyond the outer tubercles into two or three 

 branches, before they pass across the area ; all the ribs have two tubercles more or less 

 developed, and very prominent on those of the last whorl ; aperture large, oblong, 

 without processes. 



Dimensions. — Transverse diameter 135 millimetres; width of the umbilicus 55 milli- 

 metres; height of the aperture 55 millimetres; transverse diameter 50 millimetres. 



Description. — Much confusion has long existed regarding the natural history of this 

 Ammonite, so much so that nearly all the authors have mistaken Aegoceras striatum 



