22 THE VOYAGE OF H.M.S. CHALLENGER. 



Mastogloia, Thwaites. 



This genus was established by Thwaites in 1848, and is thus referred to by the late 

 Dr L. Eabenhorst, who contributed so much to our knowledge of Cryptogams, in his 

 Flora Europsea Algarum aquse dulcis et submarinae : — 



" Frustula rectangulo-oblonga, aggregata vel solitaria, valvis naviculaceis transverse 

 costatis, costis plerumque abbreviates, nodulo centrali distincto." 



Pritchard, in his History of Infusoria, defines the genus in the following words : — 



" Frustules oblong, naviculoid, annulate, in a gelatinous mammillate cushion or frond ; 

 annuli loculated ; loculi opening by foramina along the line of suture." 



The chief difference between these two definitions consists in the fact that, in the 

 former, the characteristic circumstance that the frustules are annulate and that the 

 annulus is loculate, has not been referred to. It should, however, be noted that I have 

 several times observed valves of Mastogloia entirely or partially devoid of such an annulus, 

 so that this structure must be regarded as one of unessential significance. 



Mastogloia thaitiana, n. sp. (Plate XXVI. fig. 11.) 



Valvis elliptico-lanceolatis ; apicibus productis, obtusis ; striis decussatis lineam 

 mediam simplicem attingentibus ; annulo costato totum valvse circuitum ambiente. 

 Ad oras Thaiti. 



This small form was obtained in an extraordinarily rich sounding made in the waters 

 around Tahiti. The profile is precisely similar to that of Mastogloia apiculata, 1 W. Sm., 

 but in the latter the striae are transverse, while in the former they are decussate and 

 oblique to the direction of the raphe. 



The specific name of this Diatom has reference to the locality in which it was found. 



Mastogloia kerguelenensis, n. sp. (Plate XV. fig. 11.) 



Forma elliptica ; apicibus cuneato-rotundatis ; annulo loculato, medio latiori, hinc ad 

 apices decrescente ; valvse area centralis ordine subquadrato granulata in sex fascias longi- 

 tudinales dividitur ; nodulus centralis in area lasvi inscribitur. Ad insulam Kerguelen. 



This singular and interesting species has an elliptical valve with cuneato-rotundate 

 extremities. The loculate annulus is wide in the centre, but diminishes regularly and is 

 very narrow at the ends of the frustule. The striation is transverse, subradiate, and 

 formed by lines of granules disposed in a subquadrate manner. The central area, circum- 

 scribed by the loculate annulus, is divided by a raphe and by four symmetrically disposed 

 longitudinal lines. The central nodule is represented by a smooth areola, which has an 

 indistinct outline. 



The specific name has reference to the locality in which the frustule was found. 



1 Op. cit. vol. ii. p. 65, pi. lxii. fig. 387. 



