28 PROCEEDINGS OF THE GEOLOGICAL SOCIETY. [Apr. 22, 



The original type of this species was collected by Sir Roderick 

 Murchison in the Isle of Mull. There are several fragments in the 

 collection, one showing the inner and larger whorls, in situ, from 

 which we learn that up to the third whorl the ribs are fine and nu- 

 merous, about 56 in a whorl ; in the fourth whorl they are further 

 apart ; in the fifth whorl they become more prominent, are directed 

 obliquely forwards, and as they pass over the dorsal surface they 

 form an arch, the convexity of which is towards the mouth of the 

 shell. 



This appears to be the most abundant species in the Pabba-shales, 

 as there are several specimens of different ages in the collection ; the 

 largest specimen measures 6 inches in diameter. Compared with 

 Sowerby's figure (Min. Conch, pi. 555. f. 1), the lateral ribs in the 

 specimens from Pabba are less flexed, the second whorl is dispro- 

 portionately small, and the ribs are closer together. 



Ammonites Jamesoni, Sow., is found in the Middle Lias near 

 Charmouth, and in the grey shaly clays of the same zone at Robin 

 Hood's Bay, Yorkshire coast ; I have obtained only three specimens 

 in the lower beds of the Middle Lias near Cheltenham. 



This Ammonite is found in the lower part of the Middle Lias of 

 Swabia, in many localities, as at Pliensbach by Boll, at Sondel- 

 fingen, Hechingen, Balingen, &c. The German specimens are mostly 

 fragments. In France it is found in the same region of the Middle 

 Lias at Saint Amand (Cher) and Evrecy (Calvados). As the rib- 

 bing of the shell varies at different periods of its growth, such varieties 

 have been supposed to constitute distinct species. The Ammonites 

 Regnardi, d'Orb., and the Ammonites Bronni, Roemer, are but dif- 

 ferent states of this very characteristic Ammonite from the lower 

 region of the Middle Lias. 



Ammonites brevispina, Sowerby, 1827- 



Ammonites brevispina, Sowerby, Mineral Conchology, t. 556. f. 2. 



natrix, Zieten, die Verstein. Wiirttemb. t. 4.f. 5, 1830. 



natrix, rotundatus, Quenstedt, Cephalopoda, t. 4. f. 17, 



1845. 



Sowerby's text and the numbers of pi. 556 do not agree ; this 

 mistake will mislead the reader, unless he is acquainted with the two 

 Ammonites figured in that plate. Figure 1 represents Amm. brevi- 

 spina, Sow., and Figure 2, Amm. laticostatus, Sow., as stated in the 

 description. On comparing the fragment of this species from the 

 Pabba-shales with A. natrix, Zieten, from the Middle Lias of Balin- 

 gen (Swabia), I find them to be identical ; the spines near the inner 

 edge of the whorl are nearly obsolete, those towards the back are short 

 well-developed processes, and are well characterized by the specific 

 name brevispina. The type-specimen of this Ammonite was collected 

 at Pabba by Sir Roderick Murchison, with which the fragment before 

 me entirely agrees. It is found likewise in the lower shales (Middle 

 Lias) of Robin Hood's Bay with Amm. Jamesoni, Sow. 



