ICHTHYOLOGY OF THE LESSER ANTILLES. 



453 



on 1.1) o upper side remaining rudimental, it will be necessary that such enlarged portion should strike the water in the 

 plane transverse to the longitudinal axis of the body, in order that the weight of the body be propelled with the 

 least expenditure of force. This will necessarily cause the distal vertebrae or end of the chorda dorsalis to be turned 

 upwards, so that the inferior rays of the I'm shall be brought as near to the line of the superior as possible. This is 

 the typo of tail known as the heterocercal, as called by Agassiz. 



We find among the Physoclysti that the lower rays of the (in arc more and more strengthened, and tho hffimal 

 spines which support them arc more and more enlarged ; consequently theendof the column is more curved upwards, 

 as seen in Amia. The superior rays and neural spines arc also strengthened, and the inferior so extended up- 

 wards as to pass round the extremity of (lie column, and come into contact with them. And now the vertebral cen- 

 tra are successively atrophied from the extremity. Counting from the extremity to the hasmal spines of the first sup- 

 ports of the outer rays of the caudal lin above and below, we find that ten vertebra remain in the tail of the Notop- 

 terus. In the Byodontidm Albulidm, Elopidm, AlepocephalidrnzaA Salmoniba, there are but two left, while one only ap- 

 pears in the 0steoglossido3, Aulopidm, Lutodiridm, Butyrinidm, Coregonidce, Glupeidm and CMrocentridw. In most, other 

 families, especially of Physoclysti, the last one has disappeared, and the numerous bsemal arches arc arranged like 

 radii diverging upwards and downwards from the last caudal vertebra;. In the highest groups, as Pharygognathi, etc., 

 they become coossitied, and the tail lias completed its specialization. This is Hie type called homoccrcal by Agassi/,, 

 and diphycereal by later writers. 



These types arc thus plainly stages in the development of this member, the fust and second being simply of 

 development of the last. Thus I he young Salmon commences with an cel-likc vertebral column, oris isocercal; 

 it presently, by the upward curvature of the column, and unequal development of the caudal fin becomes heterocercal. 

 Aborting tkc distal vertebra?, it makes much progress towards becoming diphycereal, but ceases to grow before il has 

 quite accomplished Ibis stage. The Polypterws, the eels, the Oymnarchus, and other fishes ossify the vertebra? in the 

 isocercal stage. The heterocercal type is seen in the Chondrostei, where the vertebras never ossify. In Lepidosteus, 

 Amid and Salmo, they ossify in tins stage. 



I will now further specify the characters of the orders of Physostomi, and the families they contain. 



Ginglymodi. Parietals in contact; pterotic simple; sympleetio present. Mandible with coronoid, opercular, 



angular, articular and dentary bones. Basis cranii simple. Third superior pharyngeal bone small, lying on fourth ; 

 upper basihyal wanting. Maxillary subdivided. A pracoracoid arch. Vertebrae opisthocoelian. Pectoral fins with 

 raesopterygium and live other basal elements. 



One family, the Lepidosteidce, with heteroceroal tail, cartilaginous pracoracoid, one axial hyoid and three basal 

 branch ihyals. 



Halecomobphi. Parietals in contact, pterotic basis cranii and anterior vertebra; simple. Mandible with oper- 

 cular and coronoid ; maxillary not segmented, bordering the mouth. Third superior pharyngeal lying on enlarged 

 fourth ; upper basihyal wanting. Vertebra', amphiccelian. Pectoral fins with mesopterygium and eight other 

 elements. 



One family, the Amiidce, with heterocercal tail, cartilaginous pnncoracoid, one axial and four basal branchihyals. 



Nematognathi. Parietals. and supraoccipital confluent. Pour anterior vertebra; coossified, and with ossicula 

 auditus. Nomesopterygium. Basis cranii and pterotic bone simple; no coronoid hone. Third superior pharyngeal 

 bone wanting or small and resting 011 the fourth, second directed backwards. One or two pairs basal branchihyals, 

 two pairs branchihyals. Subopcrciiluni wanting, prcina xillary forming mouth-border abovo. Interclavicles present. 



This division is the nearest ally to the sturgeons ( Ohondrostei) among Physostom'ous fishes, and I imagine that 

 future discoveries will prove that; il, has been derived from that division by descent. In the same way the Isospon- 

 dylous fishes arc nearest to the HalecomorpU, and h;i,ve probably descended from some Crossopterygian near the 

 Haplistia, through that order. The aflinity of the cat fishes to the sturgeons is seen in the absence of symplectic, 

 the rudimental maxillary bone, and a,s observed by Parker, in file interclavicles. There is a, superficial resemblance 



AMERT. PHILOSO. SOC. — VOL. XIV. — 114 



