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. 

 The 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 1 15 nun. in length and ISO mm. in 

 width; the smallest 6 mm in length and H) 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 

 myrmecophorus 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 Hat 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 ? acantho- 

 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 b) 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 



