No. 377-] AMIOID AND LEPISOSTEOID FISHES. 343 
neural arches. Any similar ossification on the ventral side of 
the notochord would, I suppose, be regarded as a hypocentrum. 
At least, no other elements are mentioned as occurring and 
taking part in the formation of the vertebra. It is neverthe- 
less true that other elements do exist, and in many fishes enter 
into the construction of the definitive vertebral centra. These 
elements, known in their primitive condition as intercalated 
cartilages, are present in Acipenser and in Polyodon and remain 
permanently distinct. 
In my investigations on the vertebral column of Amia,! I 
showed that these intercalated elements are of the greatest 
Fic. 1. f Fic. "ORIG. ji 
i Fic. 2. — F orapapa pir = TEE iy axis of young gon nena . ne to one rep of oe 
mid 
neural arches 
5 
At A; akt i h d of the figu re the upper h hed th l 
and support the neural arches. In this region, also, the lower intercalated aaa are 
Sees: ‘dal figu re is iS iroi a a section in the tail Sire vertebræ of the ordinary form change 
Fig. 2. — A dia anae section across one kot the See pasos See of the tail of young 
Amia “The dott ae areas re T ent cartilage ; e layer of bone covering 
the cm., the uth tik ossifications; hyp.c., he hypocentral ossifica- 
ons; z.a. EER k, h.a., hæmal arch; ochord. 
Fic. 3.—A F REN section through one of as Cada. vertebral rings of young Amia. /.c., 
pleurocentral ossi ifications ; Am., hemacentral emai Union of the four ossifications 
ill produce a complete ring. 
importance in the formation of the adult vertebral centra. I 
discovered that in the tail region there are, for each muscular 
segment, eight cartilages, viz., the bases of the two halves of 
the upper arch, the bases of the two halves of the lower arch, 
a right and a left upper intercalated cartilage, and a right and 
a left lower intercalated cartilage (Fig. 1). At a certain stage 
ossification starts on the surface of each of these cartilages. 
1 Publications of Field Columbian Museum, Zool. Ser., vol. i, No. t. 
