JAW. 57 



B. 49. Three of the six joints of the great claw (chela) of a 

 Lobster (ffomarus vulgaris). The dorsal surface of the 

 ischiopodite and meropodite is flattened, a single fulcrum 

 being formed by the entire length of the dorsal border. 



In the joint between the meropodite and carpopodite 

 there are two fulcra, each about 14 mm. long. The outer 

 fulcrum is overlapped by a process of the meropodite. The 

 joint between the carpopodite and propodite has two fulcra: 

 the dorsal one, as in the previous case, is formed by a 

 narrow line of noncalcified cuticle, by which a special 

 calcified process of the carpopodite is attached to the pro- 

 podite ; on the ventral side the fulcrum is formed by 

 thick cuticle not attached to any process. The regions of 

 both fulcra are strengthened by a hollow process of the carpo- 

 podite that overlaps the propodite. On the dorsal side the 

 process has a semicircular groove, into which fits a semi- 

 circular ridge on the propodite. On the ventral side the 

 ridge is borne by the carpopodite, the groove by the pro- 

 podite. In each the fulcrum corresponds with the centre 

 of the semicircle. The fulcra have been painted blue. 



Presented by Mr. R. Burton. 



Langer, Denkschr. d. k. Akad. Wiss. Wien, Bd. xviii. 

 I860, p. 101. 



B. 50. The dorsal and ventral portions of the joint between the 

 carpopodite and propodite of the great claw of a Lobster, 

 separated to show the special semicircular articular surfaces, 

 the ridge on the ventral being borne by the carpopodite, 

 on the dorsal by the propodite. This probably increases 

 the strength of the articulation. 



VERTEBRATA. 



JAW. 



B. 51. Joint between the right maxillary and mandibular carti- 

 lages of a Skate (Raja batis). The articular surface of the 

 maxillary cartilage is irregularly rounded, that of the 

 mandibular concave. The upper border of the latter shows 

 a notch through which protrudes a synovial pouch 8 mm 

 in diameter. The synovial membrane has a few small 

 fringes, but is apparently healthy. The cavity of the joint 



