272 IOWA GEOLOGICAL SURVEY 



SPECIAL DESCRIPTION OF THE AUDITORY ORGAN AND OTHER 



SOFT PARTS. 



BY DE. G. H. PARKER. 



The preservation of soft parts in fossils is so unusual that the specimens sub- 

 mitted to me for examination are worthy of special attention. Some of these show 

 what seem to be indubitable remains of the actual brain, internal ears, nerve end- 

 ings and blood vessels. In the specimen shown in Fig. 40 A the remains of the 

 ears are especially perfect. On the right-hand side the anterior vertical canal is 

 well preserved throughout its whole extent from the apex of the superior sinus of 

 the utriculus to the anterior ampulla. This ampulla is somewhat embedded in the 

 matrix, but is sufficiently exposed to be clearly recognizable. The posterior hor- 

 izontal canal is also observable throughout its extent, though its ampulla is not, 

 being probably concealed by matrix. Immediately behind the anterior ampulla is 

 a spherical body, distinctly shown in Fig. 40 A, which represents the ampulla of 

 the horizontal (external) canal. This canal is not visible in the particular speci- 

 men here figured, but in another that is not figured the horizontal canal is shown 

 with perfect clearness and is seen to connect with the spherical body just men- 

 tioned, thus proving it to be the ampulla. The unfigured specimen referred to (it 

 is the property of the American Museum of Natural History) also shows near the 

 posterior end of the horizontal canal a rounded body which without doubt is the 

 posterior ampulla. 



The left-hand side of the specimen shown in Fig. 40 A is almost identical with 

 the right-hand side, except that the anterior vertical canal has been partly frac- 

 tured near its junction with the anterior ampulla. In the ears of each side of this 

 specimen the apex of the superior sinus where the two vertical canals unite shows 

 a well marked depression as though the walls of the membranous labyrinth were 

 slightly invaginated. It is possible that this depression marks the spot from which 

 a well developed apex has been broken away, but the depression has the appear- 

 ance of a slight superficial involution rather than a ruptured membranous wall. 

 These two specimens together show with remarkable clearness all the essential 

 parts of the semicircular apparatus of a fish, and conform to precisely the same 

 plan as is exemplified in modern forms. 



Arteries.— Among other soft parts preserved in these specimens may be men- 

 tioned a body to be seen on the inner surface of the ventral wall of the cranium, 

 and well toward the posterior end of that structure. It is I'-shaped with the stem 

 of the Y median and directed posteriorly. The two arms of the 1' extend for- 

 wardly, and near their anterior ends they seem to divide into two unequal 

 branches. The smaller branch is directed anteriorly and probably was contained 

 within the cranial cavity, but the larger one is flexed ventrally and laterally, and 

 apparently pierced the cranial wall. At first sight this structure might be taken 

 for the remains of nerves, but its position and the relation of its parts are much 

 more suggestive of arteries. The posterior median stem corresponds to the basilar 

 artery, the large ventro-lateral branches to the carotid arteries, and the smaller 

 branches to those vessels which in fishes are distributed to the ventral and lateral 

 surfaces of the brain. It seems, therefore, that vascular as well as nervous organs 

 are preserved in these highly remarkable specimens. 



