JURASSIC. 



89 



stomach. On the other hand, one can recognize with moderate certainty the quadrant 

 of a mound-shaped ring lying on the upper left hand portion of the disk center. It 

 appears as a crescent-shaped body, everywhere 10""" broad, whose two rounded ends 

 are about 60 mm apart. It is almost absolutely certain that this is a sexual gland. 

 This is indicated by ito characteristic position and also by its crescentic shape, which, 

 as in many acraspedotes, is rounded off at both ends. Another gland, beginning near 

 the end of the first, shows that there was a complete 

 genital ring. 



The affinities of this medusa are not known with 

 certainty, since we can learn nothing of its whole 

 gastrovascular system, the form of the mouth, of 

 the stomach, of the radial canals, etc. It can be 

 asserted, however, that it belongs to the higher 

 medusae, the acraspedote or phanerocarpous medu- 

 sa?. Proofs of this exist in the marginal lobes and 

 the oral arms. Of the two groups which Agassiz 

 distinguishes in this division, the Rhizostomae and 

 the Semostomse, our fossil medusa probably belongs 

 to the former. This can be deduced with great cer- 

 tainty from the number and structure of the arms, in 

 spite, of the fact that the characteristic polystomy 

 and the lack of a central mouth, by which the Rhizo- 

 stomre are distinguished from all other medusae, are 

 not to be recognized in the obscure impression in 

 question. Most of the rhizostomes possess 8 similar 

 oral arms of a three-sided prismatic form, while a 

 similar number and structure of the arms very sel- 

 dom exists among the semostomes. Furthermore, 

 if our fossil medusa had possessed marginal tenta- 

 cles, one would expect to find at least some trace of 

 them between the lobes of the margin. But this 

 is not the case. Since these structures are charac- 

 teristically absent from the rhizostomes, while among the semostomes they are devel- 

 oped in greater or less numbers, the balance of evidence seems to be in favor of placing 

 L. trigonobrachius with the rhizostomes. Furthermore, it can be concluded, by a pro- 

 cess of exclusion, that it stands among the Leptobrachidse. 



Leuckart, in reviewing Haeckel's diagnosis of this species, states that 

 he has examined the types and can not discern the characters of a rhizos- 

 tomide, nor the 8 arms, and that he favors the views expressed in the forth- 

 coming work of Brandt. 



Dr. Brandt's observations on L trigonobrachius are so at variance with 

 those of Dr. Haeckel that he has felt it necessary to propose for the species, 



Fig. 21 — ■ Contour drawing of Leptobrachi- 



tes trigonobrachius, reduced and restored. 

 (After Brandt.) 



A, S, C, E, four distinctly recognizable 

 mouth arms; D, problematic fifth arm; M, 

 expression of the thickness of the umbrella; 

 N, lower surface of the umbrella; O, ring 

 zone, with the entrances into the genital 

 cavities; P, place of transition of the um- 

 brella into the stem; O, wall of the stem ; It, 

 transition of the stem into the bases of the 

 mouth arms ; a, boundary of the outside sur- 

 face of the umbrella; b, boundary of the in- 

 ner surface of the umbrella ; c, c', c", c'", mar- 

 ginal lobes; /, outline of the eudenteric cen- 

 tral cavity; i', lips of the mouth and side 

 plates of the mouth arms; o, o\ o'\ entrances 

 into the genital cavities. 



