LARVAL STAGES OF TRILOBITES 169 



early larval tj-pe. This would be expected from their great 

 antiquity, their comparatively generalized and uniform struc- 

 ture, and from the fact that no sessile, attached, parasitic, 

 land, or freshwater species are known. These conditions, 

 by introducing new elements into the ontogeny, would tend 

 to modify or abbreviate it in various ways, especially among 

 the higher genera. 



Before discussing any of the various philosophical and 

 theoretical pml^lems involved in an attempt to correlate the 

 larval forms of Crustacea, a brief consideration of the kno\vn 

 facts relating to the larvte of trilobites will be presented. 



^Minute spherical or ovoid fossils associated with trilobites 

 have been described as possible trilobite eggs, by Barrande^ 

 and Walcott,"- but nothing is known, of course, of the 

 embryonic stages of the animals themselves. The smallest 

 and most primitive organisms that have been detected, and 

 ti'ueed by means of series of specimens through successive 

 changes into adult trilobites, are, as stated above, little 

 discoid or ovate bodies not more than one millimetre in 

 length, as shown on Plates III and IV. It is fair to assume 

 that we ha\-e here a general exhibition of trilobite larval 

 stages, since the ten species represented are from various 

 geological horizons belonging to the Cambrian, Ordovician, 

 and Silurian sediments, with Devonian types, and showing 

 the simple as well as the highly specialized forms. 



All the facts in the ontogeny of trilobites point to one type 

 of larval structure. This is even more noticeable than among 

 recent Crustacea, in which the nauplius is considered as the 

 characteristic larval form. It is desirable to give a name to 

 this early larval type apparently so characteristic of all trilo- 

 bites, and among different genera varying only in features of 

 secondary importance. This stage may therefore be called 

 the protaspis (tt/joito? primus, aa-Trh Sfuturn). 



The principal characters of the protaspis are the following : 

 Dorsal shield minute, varying in observed species from .4 to 

 1 mm. in length; circular or ovoid in form; axis distinct, 

 more or less strongly annulated ; head portion predominating ; 



