[Illustration: FIG. 37.--Shoulder-girdle of _Sebastolobus alascanus_ Gilbert. (After Starks.) POT. Post-temporal. CL. Clavicle. PCL. Postclavicle. HYC. Hypercoracoid. HYPC. Hypocoracoid.] =The Posterior Limbs.=--The posterior limb or ventral fin (63) is articulated to a single bone on either side, the _pelvic girdle_ (62). [Illustration: FIG. 38.--Cranium of _Sebastolobus alascanus_ Gilbert. (After Starks.) V. Vomer. N. Nasal. E. Ethmoid. PF. Prefrontal. FR. Frontal. PAS. Parasphenoid. ALS. Alisphenoid. P. Parietal. BA. Basisphenoid. PRO. Prootic. BO. Basioccipital. SO. Supraoccipital. EO. Exoccipital. EPO. Epiotic. SPO. Sphenotic. PTO. Pterotic.] In the shark the pelvic girdle is rather largely developed, but in the more specialized fishes it loses its importance. In the less specialized of the bony fishes the pelvis is attached at a distance from the head among the muscles of the side, and free from the shoulder-girdle and other parts of the skeleton. The ventral fins are then said to be abdominal. When very close to the clavicle, but not connected with it, as in the mullet, the fin is still said to be abdominal or subabdominal. In the striped bass the pelvis is joined by ligament between the clavicles, near their tip. The ventral fins thus connected, as seen in most spiny-rayed fishes, are said to be thoracic. In certain forms the pelvis is thrown still farther forward and attached at the throat or even to the chin. When the ventral fins are thus inserted before the shoulder-girdle, they are said to be jugular. Most of the fishes with spines in the fins have thoracic ventrals. In the fishes with jugular ventrals these fins have begun a process of degeneration by which the spines or soft rays or both are lost or atrophied. [Illustration: FIG. 39.--Lower jaw and palate of _Sebastolobus alascanus_. (After Starks.) PA. Palatine. MSPT. Mesopterygoid. PT. Pterygoid. MPT. Metapterygoid. D. Dentary. AR. Articular. AN. Angular. Q. Quadrate. SY. Symplectic. HM. Hyomandibular. POP. Preopercle. IOP. Interopercle. SOP. Subopercle. OP. Opercle.] =Degeneration.=--By degeneration or degradation in biology is meant merely a reduction to a lower degree of complexity or specialization in structure. If in the process of development of the individual some particular organ loses its complexity it is said to be degenerate. If in the geological history of a type the same change takes place the same term is used. Degeneration in this sense is, like specialization, a phase of adaptation. It does not imply disease, feebleness, or mutilation, or any tendency toward extinction. It is also necessary to distinguish clearly phases of primitive simplicity from the apparent simplicity resulting from degeneration.