124 MEMOIRS OF THE NATIONAL ACADEMY OP SCIENCES. 



which Acanthotelson belongs, also without doubt comprises the generic and specific characters. 

 We will first give a description of the fossils themselves, based on the material we have had for 

 examination, and then endeavor to point out those characters which we suppose to be the essential 

 features of the group to which the genus belongs, and also to indicate the probable aflBnities to 

 the other divisions or suborders of Malacostraca. It may be as well to say that, after examining 

 some forty specimens, we are unable to distinguish between Acanthotelson stimpsoni and eveni, and 

 are inclined to believe that the former is the young of the latter species. 



In Mr. Lacoe's No. 501b the head is well preserved; thefli-st arthromere or segment is considerably 

 shorter than any of the succeeding ones; it is slightly less than two-thirds as long as the succeed- 

 ing arthromere; it bears in front a well-marked, small, triangular rostrum, which is acute at the 

 tip, and is about two-thirds as long as the segment itself; the edge of the rostrum is considerably 

 raised, especially at the base. The front edge of the segment on each side of the rostrum is also 

 margined with au elevated ridge. The surface of the segment is rather full and convex on each 

 side, but not so decidedly so as the second segment. The second artliromere is about as long as 

 those succeeding, though not quite so long as the sixth arthromere; on each side is a low boss-like 

 swelling, situated obliquely, and prolonged in an oblique direction to the anterior outer edge. 

 The second segment is distinctly separated by an impressed line from the first, but there is not a 

 true articulation between them, so that the first and second cephalic segments may be said to be 

 consolidated and to represent the carapace of the Schizopoda. The three succeeding segments 

 have a transverse, uninterrupted, smooth ridge situated in the middle on the third, but in the fifth 

 segment near the hind margin. The sixth and succeeding segments are smooth and even. The 

 body is of even width to near the telson. The lower edges of the segments are evenly rounded, 

 those of the hinder abdominal segments are moi'e acutely rounded. 



We have been unable to detect any positive traces of the eyes, nor can we state whether they 

 were sessile or stalked, though if they were present and sessile we do not see why they should not 

 have been preserved in some of the specimens (particularly 501" and 406").' 



The first pair of antenna} seem to arise directly from each side of the small, short, rudimentary 

 rostrum. The scape is three-jointed, and not very long and slender; second joint not so thick, 

 and about one fourth shorter than the first and twice aslong as thick ; third joint long and slender, 

 considerably longer than the second. The scape bears two tlagella, which are long, slender, mul- 

 tiarticulate branches of unequal length, of which the inner is the thicker and shorter, the outer 

 flagellum much slenderer and longer, the entire length of the antenutB being one-half that of the 

 second or outer pair. The second pair of antenna; have als6 a three-jointed scape (which is not 

 accurately represented in Meek and Worthen's figure). The basal joint is short ; second joint shorter 

 than the first, with two unequal internal spines ; third joint slightly longer than the second and 

 much smaller; there are traces of a small auteuual scale; the flagellum is long and slender, its 

 entire length about half that of the body. 



There are twelve pairs of feet (506',"), a pair to each segment situated between the head and 

 penultimate uromere or abdominal segment; these, with the caudal pair of appendages, make in all 

 thirteen pairs of legs. 



The number of arthromeres or body-segments is sixteen, counting the head as consisting of two 

 when seen from above, and the telson as a rudimentary arthromere, so that there are thirteen 

 arthromeres between the head and telson, each of them bearing legs. There is no apparent dis- 

 tinction, as regards the segments themselves, into cephalothorax and abdomen (urosome), but there 

 are two cephalic, nine thoracic segments, and seven abdominal, counting the telsou as the seventh. 

 The first seven pairs of (thoracic) legs are much alike in appearance, reminding us of those of Peta- 

 lophthalmus and Gnathophausia; these are succeeded by five pairs of abdominal appendages, which 

 are about half as long and large as the thoracic legs. The first pair of thoracic legs (which do not 

 seem to be mandibular i)alpi) are considerably larger (broader and longer) than the succeeding 

 ones. It is composed of six joints; the first and second rather narrow; the third broad, with, ac- 

 cording to Meek and Wortbeu, " three" spines on the " under side" (these were not to be seen in 

 my specimens, .though undoubtedly existing there ); fourth longer than the third, with three spines; 



'Before goin^ to press I received from Mr. Lacoea very large specimen, his No. x*, in which are two large smooth 

 concavities, one on each side of the base of the head ; it is possible that these are sessile eyes. 



