TRILOBITES. 733 
Observations on Phacopide.] 
Several convenient subgeneric names have been applied to these later forms, such as 
Odontocephalus, where the frontal limb of the cephalon bears a row of incisor-like 
processes; Corycephalus, in which similar processes extend to the genal angles; 
Coronura, where the pygidium has an echinate margin and its posterior extremity 
is erected into a semicircular collar. These names are taxonomically subordinate 
in the third degree to the term Dalmanites; that is to say, we conceive that they 
all, with the inclusion of those Devonian species referred to above as “ Chasmops” 
(forming a homotaxic group) are subsidiary to a division whose diagnostic feature 
is the more or less complete coalescence of the first and second lateral glabellar 
lobes, and for such a subgeneric division a designation is needed (e. g., Synphoria). 
The typical expression and phyletic normal of Dalmanites is represented by a 
series of upper Silurian and earliest Devonian forms, in which the glabellar lobation 
is perfect and the pygidium caudate. An excellent example is the D. limulurus 
Green, of the Niagara group. This type is foreshadowed in the lower Silurian by 
D. achates Billings, of the Trenton limestone, and perhaps by D. carleyi Meek, of the 
Hudson River group. Dalmanites achates still maintains the facial suture of Ptery- 
gometopus, with the complete glabellar lobation, anterior width of glabella and 
acuminate pygidium of typical Dalmanites. 
The acmic or mature type of Dalmanites becomes simply ornamented by 
rostrate processes on the cephalon, both in the later (Waldron) Niagara (D. bicornis 
Hall), and in the Lower Helderberg (D. nasutus Conrad, D. tridens Hall), or may 
have short triangular spines extending partially “or entirely about the margin of the 
cephalon (D. dentatus Barrett, D.dolphi Clarke). With the close of the Lower Heder- 
berg the type seems to have abruptly disappeared, but it reappeared in the Hamilton 
fauna devoid of other dermal ornament than the broad, flat marginal extensions of 
the pygidium, Crypheus. This is the last of the race in American faunas. 
These appearances are, 1 apprehend, to be interpreted and summarized as 
follows: The lobal coalesence of the early Silurian species, D. bebryx, Ch. troosti, P. 
eboraceus, is indicative of immature or epacmic development. The relation of 
Monorachus to these forms was close and probably ancestral. After passing the acmic 
period, when phyletic senility manifests itself in the variety and extravagance of 
the dermal ornamentation, the reversion to the epacmic condition of lobation is but 
an accompaniment of the decline of the series. 
Piterygometopus represents but a secondary stage in this process, a stage more 
advanced than that indicated by D. bebryx, &c. But in certain species of the genus, 
there is a lateral expansion of the first and second glabellar lobes, giving to the 
glabella as a whole a somewhat globose aspect. From such species is the point of 
