chondrocranium

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The cartilaginous cranium of an embryo before ossification.
• noun A portion of the embryonic cranium forming the bones of the base of the skull and eventually undergoing ossification.

from The Century Dictionary.

• noun A cartilaginous skull; a skull permanently cartilaginous, as that of many fishes.
• noun The cartilaginous as distinguished from the membranous portions of an embryonic skull, which may eventually become entirely bony; that portion of an osseous skull which is preformed in cartilage.
• noun In ichthyology, the persistent cartilaginous portion of the cranium occurring in many osseous fishes, such as the salmonids, subjacent to the bones.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun anatomy The developing skull, composed of cartilage, of an embryo before ossification