486 ANIMAL NATURE OF DIATOME^. 



other respect than the trapezoidal figure of its primary 

 surfaces. 



68. ZYGOCEROS. Individua libcra (?) compressa 

 utrinque corniculis doubus perforatis instructa, non con- 

 catenata. 



The two species of this genus, (Z. Surirella, Z. Rhombus) 

 were observed in a living state by Ehrenberg, who saw 

 that they were always free, arid thence established, 

 principally on the want of stipes and of cateniform con- 

 junction, their generic distinction from Biddulphia. 



He observed, also, the smooth median fascia which he 

 compared to that of the Melosirce. Having regard to the 

 rhomboidal figure of the section, and the large terminal 

 perforations of the two lateral processes, the compari- 

 son with the genus AmpJiitetras is much closer. 



In the chalk marl of Oran, I saw a fragment which I 

 could not better refer to any other genus of Diatomese. 

 It was an oblong body, with three lobes on each side, 

 and two lobes with a large circular perforation at the 

 two extremities. The space corresponding to the two 

 median and more prominent lobes, was divided from the 

 two others by means of two very distinct lines, and the 

 whole surface was regularly areolated. 



The family of Biddulphiae (Isthmia, Odontetta, Biddul- 

 phia, Zygoceros^} has affinity, according to Kiitzing, with 

 none but the following one (Angulatae) ; and in the letter 

 before referred to, he intimated to me his thoughts of 

 reuniting them to the Tripodiscieae. Here I remind the 

 reader of what I have said on the genus Amphitetras, and 

 of the family of Anguliferse, which, perhaps, excluding the 

 genus Lithodesmium, seems strictly to connect itself with 

 the Biddulphieae. If there really exist also an affinity 

 with the Tripodiscese, as Kiitzing maintains, that affinity 

 may suggest the transition to the Coscinodisceae and 

 thence to the Melosireae. 



In respect to the internal organisation of the Biddul- 

 phieae, we learn, from the fine observations of Ehrenberg, 



