418 Messrs H. and A. Adams on the genus Tyleria. 



will require to be added to the list. All these inhabit the north- 

 western portion of the Himalaya. Two of them may be found 

 in company with Alycaus strangulatus, wherever that little shell 

 occurs. B. Huttoni, Pfr., is the most local, having only been 

 taken at Simla, and at Jerripani below Mussoorie. D. folliculus, 

 Pfr., and costulata, Hutt., are abundant in the whole region. 



In conclusion, I must again advert to the total absence of the 

 Cyclostomacea in the vast plains which extend from the mouth 

 of the Indus round by the north of Delhi, and along the river 

 system of the Ganges, to the head of the Bay of Bengal, cutting 

 off from the Himalaya the central and southern mountain groups 

 of the Indian peninsula; a deficiency attributable, in all proba- 

 bility, to the want of rocky shelter, inasmuch as every variety of 

 aspect, from the most arid desert to a moist soil supporting the 

 richest tropical vegetation, is present in the circuit. It must 

 however be remarked, that in the sandstone tract, extending from 

 the west of the Sone river to Delhi, which is generally of an arid 

 character, and even where primary and igneous rocks are up- 

 heaved beneath them, as in Bundelkhund, no Cyclostomatous 

 shell has rewarded the researches of myself and others. 



Spa, Belgium, 13th November, 1854. 



XXXIX. — Description of a new genus of Bivalve Mollusca. 

 By H. and A. Adams. 



Tyleria, H. and A. Adams. 



Shell oblong, equivalve; valves thin, nearly membranaceous, 

 covered with a thin epidermis, rounded anteriorly, gaping and 

 slightly produced posteriorly. Hinge composed of a carti- 

 lage-pit in each valve, cartilage internal, ligament partly ex- 

 ternal ; a calcareous lamina extending from the cartilage-pit 

 anteriorly, as far as the front muscular scar, supported in its 

 length by calcareous septa and free anteriorly; pallial im- 

 pression with slight posterior sinus. 



The curious little shell on which this genus is founded, and 

 which we have named Tyleria fragilis , was discovered, buried in 

 sand, in the interior of other shells from Mazatlan, by R. W. 

 Tyler, Esq., to whom we have dedicated the genus, — a gentleman 

 well known as an enthusiastic conchologist and an indefatigable 

 collector. The calcareous lamella is connected with the interior 

 of the valves (for it is present in both) by means of vertical 

 plates, which, being produced on each side, cause the lamella to 

 assume a dentate appearance. 



Royal Naval Hospital, Haslar. 



