1887.] 399 [Hyatt. 



Typembryos serve to connect the earlier stages of the Neoem- 

 bryos with the true larval stages which succeed the former. Bal- 

 four and other embryologists have used the term "larva" for 

 free neoembiyos and typembryos. This term should be confined 

 to the designation of stages of growth which are immediately con- 

 tinuous with later stages and parallel, or referable in their origin 

 to the adults of allied existing or fossil forms, which are not so 

 remote as those from which the embryonic stages were derived. 



The application of such principles to the study of the younger 

 stages of fossil Cephalopoda is productive of what seems to be 

 satisfactory results. The protoconchof Owen is, according to this 

 nomenclature, the shell of the univalve veliger of the Cephalous 

 Mollusca, and a true typembryo which, though eminently charac- 

 teristic of that group, has no exact morphological equivalent 

 among adults of normal forms whether recent or fossil. 



The protoconch in fossil Nautiloidea is represented by a withered- 

 looking lump sticking to the apex of the conch in a very few ex- 

 ceptionally perfect specimens. The very general absence of this 

 lump and the presence of a scar left by its removal on the apex of 

 the conch, and the wrinkled, shrunken aspect of the lump when 

 preserved, indicate the protoconch to have had a horny texture in 

 this order. This typembryo shell must have existed among Nau- 

 tiloids with an almost unchanged aspect from the earliest Cambrian 

 (Lower Silurian) horizon until the present day, and its adult equiv- 

 alent probably existed before its appearance in Cephalopoda or 

 in the equally ancient and allied group of the Pteropoda, which 

 also had similar protoconchs. 



The true larval, or as they are here named, Silphologic 10 stages, 

 began with the formation of what Owen has appropriately called 

 the apex of the conch or true shell. Among Nautiloids this was 

 a short living chamber occupied by the bod}' of the animal, but 

 having no siphon or septum. It was completed by the deposition 

 of the apical plate, which sealed up the aperture of the protoconch 

 thus closing the opening and cutting off communication between 

 the two interiors. 



This stage can, therefore, be named the Asiphonulaor siphonless 

 larva. The apex of its conch was rounded, being built out in con- 

 centric circles from the contracted aperture of the protoconch, 



10 2iA<£t}, a grub. 



