784 Note on the Nautical Instruments of the Arabs. [Dec. 



The Trincomalee shell is interesting as an arborant form, indicating 

 the passage to Cyclostoma, which it approaches through the alate 

 species, C. petiocrianum of Gray. It appears to possess the convex 

 operculum composed of spirally concentric and exserted laminae which 

 characterizes the typical species of pterocyclos. This character seems 

 to be gradually developed in the opercula of the Cyclostomadce as the 

 umbilicus widens, and the shell becomes more discoid ; varying from 

 the flat operculum of C. elegans through that of C. involvulus, where 

 the edges of the laminae are partially disengaged, and that of C. ter- 

 mistriatum, which is thickened and shews a strong spiral keel, to the 

 convex and spirally laminar structure of the pterocycloid group. 



The species of Cyclostoma from Neemuch, described by Lieutenant 

 Hutton as No. 26, in page 520, vol. iii. J. A. S. I have ascertained 

 by a comparison of specimens to be C. semistriatum of Sowkrbt, 

 whose examples were procured from Poona. 



Query. May not the impression of a supposed species of Cirrus 

 noticed in Dr. Ward's paper on the geology of the Elephant Rock 

 in the Queda country, printed in the second part Trans. Phys. Class, 

 p. 166, be that of one of Dr. Bland's species of Pterocyclos from 

 Pulo-Susson ? A reference to the specimen No. 4 will decide. 



VI. — Note on the Nautical Instruments of the Arabs. By James 



Prinsep, Sec. 



Since the arrival of the Arab vessels which annually frequent the 

 port of Calcutta, I have made diligent inquiries concerning the in- 

 strument in use among them for the measurement of the latitude, in 

 hopes of elucidating thereby the Baron Von Hammer's translation of 

 the " Mohit" (see p. 442). I have been hitherto unsuccessful, the Eng- 

 lish quadrant or sextant having generally superseded the more ancient 

 and clumsy apparatus. One Muallim, however, seemed to recognize the 

 instrument perfectly by my description, though he could not explain its 

 construction ; and promised to bring me one on his next voyage : — he 

 stretched out his arms, when I inquired about the issabah division 

 and placing his fingers together horizontally, counted with them the 

 height of the polar star, just as I guessed must have been the early and 

 rude method of the Arab navigators. 



At length in a vessel from the Maldive islands I met with an in- 

 telligent navigator who brought me the primitive instruments with 

 which he was accustomed to work his way to Calcutta, — and as I do 

 not think they are generally known, while it is certain they are of 

 Arabic origin, I hasten to describe them as lithographed in Plate 

 XL VIII. 



