Correspondence — Dr. H. J. J. Lavis. 523 



the most interesting is the one from the Bruxellien, to which he 

 applies the name Pseudotrionyx, and classes in the Chelydridce. This 

 form is characterized by the absence of horny plates and the presence 

 of a scute-sculpture, like Trionyx, but was apparently furnished with 

 a complete series of marginal scutes, while the plastron, although only 

 imperfectly united to the carapace, is much more complete than 

 in the latter. This genus evidently forms a connecting link 

 between the typical Trionychidce and those Tortoises in which the 

 shell is complete; M. Dollo attaches more weight to the apparent 

 completeness of the marginal scutes than to the sculpture of the 

 scutes and the absence of horny plates, and therefore separates 

 the genus, together with Anostira and Apliolidemys of Leidy from 

 the Trionychidce ; but we cannot help thinking, from the variations 

 in the former respect between Trionyx and Emyda, that the argu- 

 ments are at least as strong in favour of the opposite view, and for 

 taking the presence of horny plates as a characteristic of the 

 Emydidce, in the sense in which that term is used by Dr. Sclater. 



The second genus, PacJiyrhynchus, is from the Lower Eocene, and 

 is referred to the Chelonidte ; it is represented not only by the 

 typical Belgian P. Gosseleti, but probably also by the three forms 

 from the London Clay described by Professor Sir B. Owen 

 under the names of Chelone longiceps, C. planimentum, and C. 

 trigoniceps. The genus is distinguished from Chelone by the great 

 length of the mandibular symphysis ; by the triangular form, thick- 

 ness, and slight depression of the palate ; the depth of the latero- 

 temporal notches ; the separation of the nasals ; and by the posterior 

 nares being separated from the anterior ones by long narrow 

 channels, and by opening on the posterior third of the basal aspect 

 of the cranium. These differences M. Dollo is inclined to regard as 

 of rather more than generic value. 



M. Dollo is to be congratulated on this paper, and especially on 

 the careful diagnosis of characters distinguishing all his work. 



E. L. 



coiRiraiEsiPOiisrixEnsrciE]. 



THE YOLCANIC ERUPTION OF NEW ZEALAND. 

 Sib, — Almost every geologist will have read the deeply interesting 

 accounts and speculations by Mr. Archibald Greikie and Dr. Hector 

 in "Nature," and of Mr. R. Etheridge, jun., in this Magazine. As 

 the district affected is one likely to be subjected to a careful inves- 

 tigation, may I be permitted to draw attention to a few facts of 

 important bearing on vulcanological science that should be cleared 

 up. In my paper on the geology of Vesuvius and Monte Somma 

 and in other communications, it was pointed out that after a long 

 state of quiescence of a volcano, the subsequent eruption should be 

 of the explosive or plinian type. That is to say, the stony products 

 should consist essentially of pumice due to the length of time that 



