Identity of Atops trilineatus with Triarthrus Beckii. 327 
buckler ; surface finely papillose with a row of short spines down 
the back 
In Atops we have essentially the foregoing characters, the dif- 
ference in the form of buckler which occurs in a single specimen 
being due to a very apparent cause, which likewise produces the 
different proportions of lateral and central lobes. ‘The proportion, 
orm, and every other character of the lobes of the (“abdomen’’) 
thorax and caudal shield are precisely similar. 
To those who are in the practice of examining fossils imbedded 
in stone, these differences of form and proportions are readily un- 
derstood, and to those who may rely upon such characters for 
Specific distribution, we can only say that following such a rule, 
the number of fossil species can easily be quadrupled in a short 
time without the necessity of further investigations. 
he determination of a species and of its importance particu- 
larly in connection with geological questions, is a matter of some 
consequence, and we must protest against such a course as will 
place paramount to every other decision, the determination of 
two supposed species of distinct genera, by authority, however 
competent, from the examination of a single imperfect specimen 
of each, imbedded in a material of different character, and with 
teference to no other characters than difference of form and pro- 
portions of parts of the individuals. 
The reasons for referring the genus Triarthrus to Calymene 
are mainly founded on the structure of the cephalic shield, the 
maxillary portions of which are shown in fig. 2%, 
where it will be seen that the facial suture termin-_ 
ates at the posterior angle of the buckler, leaving a v 
Portion of the base of the eye upon the maxillary 
shields. The labrum or epistoma shown in con- 2i 
hexion, is that of a true Calymene, and though I have never 
found it attached to the fossil, it occurs with the other fragments 
mm such positions as to leave little doubt of its relations. In the 
Olenus, the body of which resembles the Triarthrus, the facial 
_ Suture terminates at the basal margin of the cephalic shield dis- 
tant from the angles. The direction of the suture in front of 
the glabella and the form of the eyes are also quite different from 
the Triarthrus or Calymene. 
