Larval Stages ofTrilohites. — Beerher. 169 
Devonian types, and showing the sinijile 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 2}>^ofa}tp/s (^npcoTOS, primus; affnis, scutum). 
The principal characters of the protaspis are the following: 
Dorsal shield minute, varying in observed species from A to 
1 mm. in length ; circular or ovoid in form ; axis distinct, more 
or less strongly annulated ; head portion predominating; 
glabella with five annulations; abdominal portion usually less 
than one-third the whole length of the shield, axis with from 
one to several annulations ; pleural portion smooth or grooved ; 
eyes when present anterior, marginal or submarginal ; free- 
cheeks when present very narrow, marginal. 
Several moults took place during this stage before the com- 
plete separation of the pygidium or the introduction of tho- 
racic segments. When such moults are recognized, they may 
be considered as early, middle and late protaspis stages, and 
designated respectively as anaprotaspis, metaprotaspis and 
paraprotaspis. They introduced various changes, such as the 
stronger annulation of the axis, the beginning of the free- 
cheeks, and the growth of the pygidial portion from the in- 
troduction of new appendages and segments as indicated by 
additional grooves on the axis and pleura. Similar ecdyses 
occur during the nauplius stage of many living Crustacea be- 
fore a decided transformation is brought about. Certain of 
these later stages have received a distinctive name, and are 
called the metanauplius. 
It is believed that the protaspis is homologous with the 
nauplius or metanauplius of the higlier Crustacea. Most of 
the reasons for this ])elier will appear later in the present 
paper; some wiiich may be stated now are as follows: 
(1) The size of the protaspis does not difi'er greatly from 
that of many nauplii, and represents as large an aninuil as 
could be hatched from tlie bodies consitlered as the eggs of 
trilobites. 
