CRUSTACEA. 
39 
pair of sharp tubercles in the median depression between the lateral lobes. 
The occipital furrow is narrow, and the occipital ring broad and rounded. 
Tiie entire surface, except in the furrows, is sharply tubercled. 
Dimensions. An average pygidium measures 50 mm. in axial length and 
80 mm. in width; the largest observed is 115 mm. in length and 180 mm. in 
width; the smallest 6 mm in length and 10 mm. in width. The glabella 
described has a length of 33 mm. and about an equal width. 
Observations. The remarkable size attained by the pygidia of Dalmanites 
myrmecophorits show it to have been, at maturity, an animal of gigantic propor¬ 
tions. A careful restoration of the outline of an individual on the basis of 
the largest pygidium above referred to, has been made by comparison with 
the proportions of other species of this type of Dalmanites, and the result, which 
may be regarded as approximately correct, at all events not exaggerating the 
size, indicates that the animal when entire may have attained a length of 398 
mm. or 16 inches, the largest form of Dalmanites yet observed, and one of the 
largest trilobites known. A similar restoration of the smallest pygidium 
gives an individual 21 mm. or .8 of an inch in length. Fragments of large 
cephala and thoracic segments have been occasionally observed in association 
with these pygidia, but our present knowledge of these parts is insufficient for 
satisfactory description. A few thoracic segments found in place with a pygi¬ 
dium indicate an irregularly nodose surface, and fragments of the head-shield 
show a wide and flat border, and a closely and strongly tubercled surface. The 
differences and similarities in Dalmanites myrmecophorus and Dalmanites aspectans 
are noticed under the description of the latter species. The Asaphus ? acontho- 
pleurus of Conrad {loc cit.) was described from a fragment of the posterior portion 
of a very large pygidium of this species, in which the central spine is very 
strong, and the other marginal spines, as indicated by their bases, larger than 
have been observed in any other example. This specific term has been used 
in the various preliminary publications upon these fossils, as the nature of the 
posterior border in Dalmanites myrmecophorus had not at that time been observed. 
There now appears to be no doubt of the identity of the two species. The 
