220 



afterwards the tubes are added irregularly ; sometimes we find speci- 

 mens which send off four, others five branches from the different pri- 

 mary tubes. This Medusa attains a size of several inches in diameter 

 — from three to four inches. Specimens measuring about one inch in 

 diameter have only the four principal tubes. I think there can be no 

 doubt from the figures of Berenix and Cuvieria, given in the illustrated 

 edition of Regne Animal of Cuvier, that this genus, which I would 

 call Halopsis, is closely allied to Berenix. The disc of this Medusa is 

 perfectly colorless ; the lips are short, generally carried in four simple 

 lobes, as in Oceania (Thaumantias Auct). The eyes of this species are 

 large, the tentacular cirri long, and the principal tentacles very much 

 thickened at base. The ovaries do not extend quite to the circular 

 tube from the base of the digestive cavity. For this species I would 

 propose the name of H. ocellata. I may also add here that Prof. 

 Agassiz has satisfied himself, after having examined this Medusa, that 

 the Berenicidoi of Eschscholtz and the Williadce of Forbes are two dis- 

 tinct famihes, and should not be united, as they have been in his tabu- 

 lar view referred to above. 



Prof. Jeffries Wyman made the following observations 



On the Mechanism of the Tibio-tarsal Joint of the 

 Ostrich. 



In the ordinary locomotion of the Ostrich, the tarsus as it passes 

 from a state of flexion to that of extension, or the opposite, com- 

 pletes its movement with a sudden jerk. The mechanism of this has 

 been studied by Sir Charles Bell.* According to his explanation, the 

 jerking motion depends upon the tension of one of the lateral liga- 

 ments, caused by its rising over a tuberosity on the inner side of the 

 condyle of the tibia. This tuberosity forms a double inclined plane ; 

 and as the tarsus moves backwards and forwards, the ligament, as it 

 slides over the bone, becomes tense when it reaches the summit of the 

 tuberosity, which would be about midway between flexion and exten- 

 sion ; passing this point, it slips with a jerk into the groove before or 

 behind the tuberosity. In this account no notice is taken of the form 

 of the articulating surfaces. 



From a recent dissection of an ostrich, we are satisfied that the tu- 

 berosity makes no essential difference in the tension of the ligament, 

 having but a very slight elevation, and that in the extreme positions 

 the Hgament does not slip into the grooves as stated by Sir Charles 

 Bell. 



According to our observations, the tension of the ligament depends 



* The Hand. London, 1854. p. 94. 



