Ammonites. 



MOLLUSCA. 



15 



66. A. PERARMATUS — The Well-armed Ammonite, pi. XI. 

 fig. 2. 



Ammonites perarmatus. Sowerby, Min. Concli. p. 72, 

 pi. 352. 



Discoidal, compressed ; four exposed volutions, armed with 

 two concentric series of acute tubercles ; these are transversely 

 united by eighteen or twenty pairs of obtuse ribs upon each 

 volution ; the remaining portion of the shell is plain and 

 flat ; ambit rounded ; aperture longer than wide, almost 

 orbicular, and occupying in lengtii about one-third of the 

 diameter of the disk. Greatest diameter eight and a half 

 iuclies. 



Found in the Coral Rag at Malton, in the Coral Oolite, 

 Calcareous Grit, and Kelloway's Rock, Yorksliire. 



67. A. Smithi. — Smith's Ammonite, pi. XI. figs. 4, 5. 

 Ammonites Smithi. Sowerby, Min. Conch. IV. p. 148, 



pi. 406, figs. 1, 2, 3, 4. 



Discoidal, compressed, carinated ; five nearly wholly ex- 

 posed volutions, crossed by many slightly curved distant 

 ribs ; sides flattened ; keel obtuse ; aperture oblong-ovate. 

 Greatest diameter two inches. 



The surface of this species exhibits a beautiful pearlaceous 

 lustre. It is subject to great variety in its aspect from the 

 young to the adult state. In its very young stages, it is 

 nearly globose, entirely plain, and umbilicate, as in fig. 3 ; as 

 it increases in growth, its ribs are gradually developed, first 

 in the form of somewhat elongated tubercles, near the margin 

 of the umbilicus ; soon after this, the keel appears, and it 

 then approaches its mature state, with the flattened shape 

 and fully formed ribs, as in fig. 4. 



Found associated with Ammonites planicosta, pi. Vlt. fig. 7, 

 in the dark Marly Limestone, called Marston Stone, at 

 Marston Magna, near Ilchester, and also at Evershot, Somer- 

 setshire. It bears some resemblance to that species, but is at 

 once distinguished by the ribs being destitute of the tubercu- 

 lated sharpness of that shell. 



Named in honour of W. Smith, Esq. an excellent geologist, 

 and author of a Geological Map of England, &c. 



68. A. FiBui-ATUs. — The Button and Loop Ammonite, 

 pi. XI. fig. 5. 



Ammonites armatus. Young and Bird, Geology of York- 

 shire, p. 250, pi. 13, fig. 9. A. Jihidatus. Sowerby, Min. 

 Conch. IV. p. 147, pi. 407, fig. 2. 



Discoidal, much compressed ; six volutions, the inner ones 

 almost entirely exposed, flattened on their sides, with their 

 inner margins plain ; traversed by numerous transverse ribs, 

 which are joined in pairs by smooth spines ; these increase 

 the uncommon flatness of this species, by filling up at inter- 

 vals the suture which separates the volutions ; there are 

 placed at unequal distances ribs which pass over the dorsal 

 margin without uniting with the spines ; these are more 

 numerous in the external volutions, and in some instances, 

 alternate with the spines ; each rib, when disengaged from 

 the spine, is distinctly separated into two, consequently a 

 greater number of costas pass over the back than are found 

 upon the sides of the shell ; aperture oblong. It is distin- 

 guished from Ammonites armatus, pi. VIII. fig. 2, by its 

 smooth spines. 



Found in the Lias, at Whitby, &c. Yorkshire. 



69. A. suBARMATUs The Subarmed Ammonite, pi. XI. 



fig. 1. 



Ammonites subarmatus. Young and Bird, Geology of 



Yorkshire, p. 250, pi. 13, fig. 3 ; Sowerby, Min. Conch. IV. 

 p. 146, pi. 407, fig. 1. 



Discoidal, compressed, concave ; six volutions, the inner 

 ones almost entirely exposed ; sides crossed bj' slightly curved 

 ribs, whicli are frequentl3' united in pairs by smooth spines 

 at their exterior extremities, continuing to nearly the central 

 volution, but on the outer volutions these generally disap- 

 pear; and the ribs, which, in the earlier volutions, are usually 

 divided before they pass over the back, are but seldom split. 



The spines are hollow, the sides of the volutions are 

 convex and increase rather rapidly in size, but are, like 

 the Ammonites mutabiles, pi. XIII. fig. 7, a little constricted 

 in some places. 



Found in the Lias, at Whitby, Yorkshire. 



70. A. Davjei — Davy's Ammonite, pi. XL fig. 7. 

 Ammonites Bavcei. — Sowerby, Min. Conch. IV. p. 70, 



pi. 350. 



Discoidal, compressed ; about five entirely exposed volu- 

 tions, their sides almost flat, with very numerous transverse, 

 slightly arcuated ribs, the interstices between them being 

 nearly equal to their breadth, with a series of remote obtuse 

 tubercles in the centre of the volutions, each covering about 

 four of the sulci ; aperture nearly orbicular. Found in the 

 Blue Lias at Lyme Regis, Dorsetshire. 



Named in honour of the distinguished Sir Humphrey Davy. 



71. A. L^viGATUs — The Smooth Ammonite, pi. XL fig. 8. 

 Ammonites Imvigatus. Sowerby, Min. Concii. VI. p. 1 35, 



pi. 570, fig. 3. 



Discoidal, compressed, smooth ; three convex volutions, 

 the inner ones exposed ; aperture transversely oblong-ovate, 

 with a thickened margin, somewhat produced in front. 



Found in the Lias, at Lyme-Regis. 



72. A. PARVUS — The Small Ammonite, pi. XL fig. 9. 

 Ammonites parvus. Sowerby, Min. Conch. V. p. 70, pi. 



449, fig. 2. 



Discoidal; four or five~ gradually enlarging volutions, the 

 inner ones exposed ; sides furnished with numerous, elevated, 

 obtuse, divergent, undulating strias ; ambit considerably 

 rounded ; aperture ovate, occupying one-third of the diameter 

 of the disk. 



Discovered in the shaft, while sinking a well at Tunbridge, 

 eighty feet below the surface. 



73. A. CRisTATUs. — The Cristed Ammonite, pi. XL fig. 10. 

 Ammonites cristatiis. Defrance, MSS. ; Sowerby, Min. 



Conch. V. p. 24, pi. 421, fig. 3. 



Lenticular, compressed, carinated ; inner volutions con- 

 cealed, the outer one increasing rapidly ; keel thin, largely 

 and deeply notched. 



Found at Weymouth by Mr Bryer. 



74. A. Bakebi.b. — Baker's Ammonite, pi. XI. fig. II, 

 and 14. 



Ammonites Bakerite. Sowerby, Min. Conch. VI. p. 134. 

 pi. 570, figs. 1, 2. 



Discoidal, compressed ; four volutions, inner ones exposed ; 

 sides with transverse, numerous, furcated, armed ribs, and 

 about ten or eleven sinuous lines, — which are its distinguishing 

 characteristic, — extending over the back ; aperture oblong- 

 ovate. 



Discovered by Miss Baker in an indurated nodule of 

 Marie amongst Alluvium, parish of Brauuston, Northamp- 

 tonsliire, and named in honour of that lad}'. 



75. A. Herveyi. — Hervey's Ammonite, pi. XL fig. 12. 



