interdigitated in large specimens. The largest element of 

 the caudal skeleton is that formed from the fusion of the 

 centrum of the last vertebra posteroventrally with the 

 lower hypurals, which in generalized plectognaths would 

 correspond to the first and second hypurals but which are 

 here indistinguishably fused with one another and with 

 the centrum. The concave anterior face of the centrum 

 portion of this fusion element articulates in a normal 

 manner with the concave posterior face of the centrum of 

 the penultimate vertebra, but immediately below this 

 fibrous tissue articulation the ventral expansions of these 

 two vertebrae interdigitate with one another, while ven- 

 trally that of the last centrum interdigitates with the 

 anterodorsal end of the parhypural, except at the medial 

 longitudinal groove between their apposed surfaces 

 enclosing the haemal canal. The urostyle of the last cen- 

 trum is represented by a tapering thickened shaft ex- 

 tending posterodorsally from the centrum portion of the 

 composite plate and is extensively interdigitated along 

 its posterior edge with the anterior edge of the autogenous 

 upper hypural plate. The anterior edge of the urostylar 

 region articulates by fibrous tissue, and by interdigita- 

 tion in larger specimens, with the epural. The ven- 

 tral edge of the hypural region of the fused centrum- 

 lower hypurals articulates by fibrous tissue with the 

 posterodorsal edge of the parhypural, while its dorsal 

 edge is extensively interdigitated with the anteroventral 

 end of the autogenous upper hypural plate. The upper 

 hypural plate is composed of indistinguishably fused hy- 

 purals corresponding to what in generalized plectog- 

 naths would be the third to fifth elements, the plate be- 

 ing extensively interdigitated with the urostylar and 

 hypural regions of the fused centrum-lower hypural 

 plate, and held for a short distance anterodorsally by 

 fibrous tissue to the epural. In the smallest specimen ex- 

 amined, 10.0 mm, the lower hypurals are as fully and in- 

 distinguishably fused to one another and to the centrum 

 as in adults, while the autogenous upper hypural plate, 

 the epural and parhypural are, as expected, scarcely if at 

 all interdigitated with the fused centrum-lower hypurals, 

 but the upper hypural plate more so than the epural and 

 parhypural. 



Caudal fin rays. — Eleven in number, the upper- 

 most ray and the lowermost ray unbranched, the others 

 becoming increasingly branched toward the middle rays, 

 which are branched in triple dichotomies. The upper- 

 most five rays are held by fibrous tissue to the auto- 

 genous upper hypural plate, the lowermost ray to the 

 parhypural and the upper five rays of the lower lobe to 

 the hypural region of the fused centrum-lower hypural 

 plate. 



DORSAL AND ANAL FINS. 



Dorsal Fin. 



Fin rays and pterygiophores. — Ten fin rays are pres- 

 ent in most specimens, the first one or two rays unbranch- 

 ed, the others branched in single or double dicho- 



tomies; the first ray almost as long as the longest ray. 

 Distal pterygiophores either absent or unossified. The 

 fin rays are supported basally by eight basal pterygio- 

 phores which are broadly interdigitated with one another 

 and which decrease slightly in length posteriorly in the 

 series. The first basal pterygiophore is by far the largest 

 and fills much of the space between the neural spines of 

 the seventh and eighth abdominal vertebrae. The second 

 to fourth basal pterygiophores have their ventral or in- 

 temeural portions between the neural spines of the 

 eighth abdominal and first caudal vertebrae, while those 

 of the fifth to seventh basal pterygiophores are between 

 the neural spines of the first and second caudal verte- 

 brae. The eighth basal pterygiophore has its ventral 

 region placed just behind the upper end of the neural 

 spine of the second caudal vertebra. The pterygiophores 

 articulate with the neural spines by fibrous tissue. Each 

 pterygiophore ends dorsally in a short columnar projec- 

 tion whose deeply concave dorsal edge is cartilage filled. 

 It is to these columnar projections that the fin rays are 

 held by fibrous tissue. The ventral ends of the pterygio- 

 phores are all deeply cartilage filled. The anterodorsal 

 edge of the first pterygiophore is slightly prolonged 

 anteriorly and articulates by fibrous tissue with the 

 rounded shaftlike supraneural. The supraneural extends 

 from above the posterior region of the neural spine of the 

 fourth abdominal vertebra to just in front of the dorsal 

 end of the neural spine of the seventh abdominal verte- 

 bra. The supraneural is connected with a band of longi- 

 tudinal muscles in the midline of the body which 

 anteriorly connects with the supraoccipital spine. None 

 of the pterygiophores possess lateral flangelike projec- 

 tions along their lengths for muscle attachment, 

 although they are slightly thicker in the middle regions 

 throughout their lengths than they are anteriorly or pos- 

 teriorly. 



Fin rays and pterygiophores. — Nine fin rays are 

 present in most specimens, the first one or two rays un- 

 branched, the others branched in single or double 

 dichotomies; the first ray only slightly shorter than the 

 longest ray. Distal pterygiophores either absent or un- 

 ossified. The anal fin rays are supported through fibrous 

 tissue by five basal pterygiophores which are interdigi- 

 tated with one another only for a short distance ventral- 

 ly. Posterior to the enlarged first pterygiophore, the re- 

 maining pterygiophores decrease only very slightly in 

 length posteriorly in the series. The dorsal ends of the 

 pterygiophores articulate by fibrous tissue with the ven- 

 tral edges of the haemal spines of the caudal vertebrae, 

 with the first pterygiophore attached to the first caudal 

 vertebra and the remaining pterygiophores attached to 

 the second and third caudal vertebrae. Like the pterygio- 

 phores of the dorsal fin, those of the anal fin are carti- 

 lage filled proximally, while distally they possess short 

 columnar projections whose deeply concave edges are 

 cartilage filled. Whereas the second to fourth pterygio- 

 phores possess a single columnar projection distally, the 



