Values of the Stages of Growth and Decline. 875 
Typembryos serve to connect the earlier stages of the Neoem- 
bryos with the true larval stages which succeeded the former. 
Balfour and other embryologists have used the term “larva” for 
free neoembryos 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 seem to be 
satisfactory results, The protoconch of 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 excep- 
tionally 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 pre- 
served, indicate the protoconch to have had a horny texture in this 
order. This typembryo shell must have existed among Nautiloids 
with an almost unchanged aspect from the earliest Cambrian (Lower 
Silurian) horizon until the present day, and its adult equivalent 
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’ 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 body of the animal, but having no 
Echinodermata. This stage, however, was not named in the address 
above quoted, which was intended as preliminary to an illustrated essay 
on the same subject, and Mr. Agassiz has supplied that omission in the 
following note, which I quote from a letter to me. “I intended some- 
time when revising my ‘ Address on Pal tological and E yological, 
Development,’ to call the earliest tag derm embryos 
; Echinula’ for convenience in making comparisons.—A. Agassiz.’ 
Zion, a grub. of 
g wW 
