ANIMALS. 133 



area equal to the width of the shell ; slowly decreasing in height from the centre to 

 the terminations, which are bluntly tapering : surface covered with regular imbricated 

 lamellae of growth, which, in passing over the ribs, appear as if undulating : incremental 

 lamella very finely striated in the cross direction, which results from the microscopic 

 capillary-like fibres constituting the substance of the shell. Small valve two thirds the 

 length of the large valve, and similarly ribbed : median elevation moderately prominent : 

 area low : umhone slightly prominent : punctures extremely small. 



I strongly suspect that the fossil which de Verneuil has doubtingly identified with 

 the Terehratulites hystericus of Schlotheim, and the " espece indeterminee" of the same 

 author are referrible to the present species. 



Trigonotreta undulata (as well as the last species) is prettily ornamented with 

 regular imbricated laminae of growth crossed with fine striae, which, when examined 

 with a magnifier, are seen to resemble capillary fibres regularly arranged in parallel 

 series, as represented in fig. 1 7, PI. IX. 



This species occurs in the compact limestone at Midderidge (Sowerby) ; in the 

 shell hmestone at Tunstall Hill, and Humbleton Q-uarry ; and in the Breccia at 

 Tynemouth Cliff. It has not yet been noticed in Germany, owing probably to its 

 being confounded with Trigonotreta alata. The " espece indeterminee, and the so- 

 called ' Spirifer hystericus T are from the Permian rocks of Russia ; the former from 

 Bielebi (government of Orenbourg), and the latter from Kirilof. 



Trigonotreta Permian a, King. Plate IX, figs. 18, 19, 20, 21, 22, 23, 24. 



Spibifer Permianus, Kinff. Catalogue, pp. 7-8, 1848. 



Diagnosis. — Margin "semi-elliptical. Twice as wide as long. Lateral surfaces \\\\ki. 

 four or more sharpish, rather distant ribs. Mesial furroio or ridge not much larger 

 than the adjoining folds. Beak erect"^ in casts ; but gibbous in testiferous specimens. 

 Valves marked with regular lamellae of growth, crossed with hair-like striae. 



This well-marked species, which rarely exceeds an inch in width, and half an inch 

 in length, differs from Trigo?iotreta undulata in having a narrower median furrow, or 

 elevation, and only half the number of folds, which are broader and more angulated 

 than those of the latter: the valves are not so tumid, and the lateral extremities 

 are rounded instead of pointed. These distinctive characters are equally well dis- 

 played on young as on old specimens. The valves are marked with lamellae of 

 growth and punctures agreeing with those of the last two species. 



I have seen in Mr. T. Davidson's collection a Belgian fossil, which, if I mistake 

 not, Dr. de Koninck has identified with Trigonotreta cristata, Schl. : it has a closer 

 resemblance, however, to the present species ; but it differs in having large punctures. 



^ King, Catalogue, pp. 7-8. 



