Miscellaneous. 75 



To each of these two principal modes is to be referred the deve- 

 lopment of larval forms. Here, in fact, we find an application of 

 the principle lately affirmed again with so much authority by M. 

 Milne-Edwards, that the larva is onJy an emhrijo with independent 

 existence. The development of the Pilidium and of the larva of Desor 

 shows us that the Pilidium is only the free representative of one of 

 the stages existing in our first embryonic type (the Gastrula-?>ta.^e 

 at the moment of the origin of the four invaginations of Miiller), 

 whilst the larva of Desor is the representative of one of the stages 

 of the second type (embryo composed of an interior mass [middle 

 lamella, endoderm] and of a superficial layer [exoderm]). The Pili' 

 dium and the larva of Desor consequently cease to be enigmatic 

 forms, opposed by the complexity of their structure to the simplicity 

 of organization of the group ; they enter with perfect right, and in 

 the most natural manner, into the normal cycle. 



The complete parallelism between the different modes of develop- 

 ment at the same time teaches us to see in the fall of the super- 

 ficial membranes in the Pilidium and the types which belong to it 

 nothing more than a mere anomaly due to the exaggerated deve- 

 lopment of the exoderm. The return to the normal condition which 

 must follow the formation of the internal organs necessarily produces 

 an inverse phenomenon ; and it is to this inverse phenomenon that 

 we must ascribe the cases of breaking-down (Balanoglossus) or de- 

 struction of the skin (Nemertes). 



These different points show that in the organization of the Nemer- 

 tians there is an essential feature around which all the embrt/ological 

 phenomena group themselves — the division into cephcdic muscidature 

 and musculature of the body. This fact has its analogues in the Tur- 

 bellaria (Stenostomum, Leucops) ; it is in complete opposition to the 

 origin of the musculature in the Annelida. Embryogeny, therefore, 

 contrary to what has hitherto been supposed from the complexity of 

 the larval forms, leads to an approximation to the lower Vermes and 

 the Turbellaria, rather than to the higher Vermes and the Annelida. 

 — Comptes Eendus, April 10, 1876, p. 859. 



Supposed Latirentian Fossil. 



To the Editors of the Annals and Magazine of Natural History. 



Gentlemen, — In the May issue of the 'Annals ' appeared a letter 

 from Mr. James Thomson and myself, in which we announced the 

 discovery in the Laurentian Rocks of Harris of what we believed to 

 be unequivocal organic remains. This opinion was based partly 

 upon the remarkable characters exhibited by microscopic sections, 

 and partly upon tbe too confident belief that the specimens were 

 essentially calcareous in their composition. This latter belief not 

 only formed one of the chief elements on which our conclusion was 

 founded, but strongly wei<i:hed with Dr. Carpenter in his indorsement 

 of our original views (' Nature,' May 4th). I wish, however, now 

 to be allowed to state frankly that our opinion as to the calcareous 



