82 coNTinr.rnoxs TO NORTH AMERICAN ICHTHYOLOGY iv. 



SUBCLASS CHONDROSTEI. 



(The Xturgeon*.) 



Skeleton ]i;irtly cartilaginous. Ventral fin abdominal, with an entire 

 Krriesofbasilar segments. Xosubopereuluiii or preoperenlum. liranchi- 

 -mgle or) wanting; a pnecoracoid arch ; no symplectie bone. 

 Mesopten giiim distinct; interdavicles present.* Arterial bulb with 

 several pairs of valves. Optic nerves forming a chiasma. Intestine 

 with a spiral valve. Air-bladder connected by a duet, with the tesopha- 

 -u>. Tail heterocercal. its tin with fulcra. Skin naked or armed with 

 bony plates. This group comprises two orders. 



(OnlT i'ln>ili-t>xlii (.iintluT, viii, li^-olT. #6i"5pof, cartilage; uari<<r, Lone.) 



ANALYSIS OF ORDERS OF CIIONDROSTKI. 



* Maxillary and intrrnpcrrlc obsolete; skin naked; branchiliyals cartilaginous; air- 

 lila.NliT ci-llular ............................................. SELACIIOSTOMI, G. 



' ' .Maxillary and iiitn opn i 1.- jtresent ; skiu with bony shields ; braiichihyals osseous ; 

 air-liladdiT sini])lc ............................................ GI.AMOSTOMI, H. 



ORDER G.-SELACIIOSTOMI. 



(The raddlc-Jishcs.) 



A |>r:ecoraeoid arch ; no sympleeticbone ; i)remaxillary forming mouth. 

 border; no snboperciiliiin, i)reoperculum, nor interopereulum ; mesop- 



distinct ; basihyals and superior ceratohyal not ossified: inter- 

 cla\ides present: maxillaries olisolete; branchihyals cartilaginous. 

 Tlii- order contains but one family, Polijinlontltla'. (<T:/.u-/j n shark ; <rr/^a, 



mouth.) 



.FAMILY X XV. POLYODONTID^. 



(The ]'(i<i(ll<'-jislics.) 



r.ody fiisit'oi-iii. little compressed, covered with mostly smooth skin. 

 Snout prolonged, expanded into a thin, Hat blade, the inner portion 

 formed by Hie produced nasal bones, t lie outer port ion with a reticulate. 

 hon\ l'rame-\vork. the \\holc somewhat llexible. IMontli broad, termi- 

 nal, but overliiuiu by the spatnlate snout, its border formed by the pre- 

 maxillaiie-. tin- maxillaries being obsolete. ,la\\s \\ ith many linedecid- 



'I I.. .,,1, ,,!,,- i,-; ;i 1 i-liarai ten ln-n- and I-!M- \vhcrc in this \\nrK arc mostly taken from 

 1 iiirilniiioii to tin- I. ljtli\.il,.-y (l f tlic> LOM-I -Anlilli > ". Trans. Am. Phil. 



S..c. 1- 



