MULTILOCULAR SHELLS. ^49 



Sowerby's shell. We must therefore sui)pose, that the latter is so 

 rare, that it could not be found among all the thousands of spe- 

 cimens which we have examined; or that Mr. Sowerby has been 

 misinformed as to its locality; or, which seems most likely, that he has 

 made out his figure by combining two specimens together, of which 

 the outer one has not belonged to our alum shale, and the inner one 

 has had some of its knobs altered, to connect it with the other. No. 9, 

 Pi. XIII, is the inside of Sowerby's a. annatus, with a little variation 

 in the knobs, which are not striated but plain, and rather sharp. The 

 specimen appears to be nearly entire, and we have no reason to think 

 that it has had another whirl, like that in Sowerby's figure. Yet in 

 tracing the spire in its several volutions, we see, as in his shell, a succes- 

 sion of different markings. Towards the mouth, the ribs are alternately 

 knobbed and plain, and the knobbed rib parts into three at the back, 

 immediately beyond the knob, while the plain rib goes round the back 

 single. At the distance of half a whirl from the mouth, the single 

 ribs disappear, and each knob is connected with a double rib, resemb- 

 ling a loop attached to a button; each double rib passing into three 

 or four on the back. This kind of marking, common to most of our 

 knobbed ammonites, continues inward through the next two whirls ; 

 beyond which, the knobs gradually disappearing, the interior part of 

 the shell is plain, and fine ribbed. This part greatly resembles small 

 specimens of our a. tenuicostatus ; but the latter is a distinct species, 

 growing to a considerable size. We have one entire specimen, above 

 three inches broad, with part of the reflected lip. 



No. 1 1, PI. XIV, is a shell with similar markings, but of a species 

 quite distinct ; for the spire, instead of being nearly rounded, appears 

 as if greatly compressed from the back inwards, and the interior 

 shews a deep cavity on both sides, the outer whirls forming a high 

 border around it. Besides, the button and loop marking does not 

 give place to plain ribs in the interior part, but continues towards the 



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