On the structure and classification of the Treiiataspidae. 3 



of a whole summer. I considered my self fortunate, therefore, in being able to purcliase 

 almost the entire collection of fossils made by Herr Simonson during that season. The 

 collection contained many specimens of Thyestes, Cephalaspis and Bunodes, together with 

 four heads of Tremataspis, some of which were in exceptionally good condition, but unfor- 

 tunately none of the last named genus showed the presence of the plates in the oral region. 



All these specimens have been added to the collections in Butterfield Museum of Dart- 

 mouth College. The material representing the first three genera will be made the subject 

 of special memoirs at a later period. 



The four heads of Tremataspis, which have been cleaned with great care, enabled me 

 to make out several new details concerning the sensory openings on the dorsal surface; they 

 were the first to show the system of lateral line pits, although these pits were afterwards 

 seen on most of the St. Petersburg specimens. But they were of special value in reconstruct- 

 ing the anterior margin of the dorsal shield, and in discovering the three new marginal 

 openings continuing forwards tlie series of six so called gill openings described by Rohon. 

 These parts, in the Petersburg material were absent, or had been destroyed by rough 

 handling. 



The reconstruction of the oral region was made from a study of the single specimen 

 and its cast, that has already been described by Rohon. The original fossil had lost many 

 details through repeated handling and the apparently incautious attempts to clean out the 

 matrix between the edges of the plates. The original cast, however, was in nearly perfect 

 condition, and by taking several impressions of it in dentist's wax, a beautiful leproduction 

 of the original untouched fossil was obtained, from which were worked out all the details in 

 the arrangement of the oral plates shown in the restoration. These casts and the enlarged 

 model are now preserved in Butterfield Museum at Dartmouth College. 



