10 PROF. T. JEFFERY PARKER ON THE 



The most noteworthy point about the exoccipitals is their union with one another 

 below the foramen magnum. In this particular, Regalecus agrees with the Cy- 

 prinoids \ 



The epiotics {ep.o). — Immediately in front of, and above the exoccipital, comes 

 another pair of bones occupying the positions usually taken up by the supraoccipital, and 

 united to one another by a narrow strip of cartilage, which broadens out at its hinder 

 end, forming the upper boundary of the occipital foramen (fig. 10). Each of these 

 bones has an irregularly oval outline, articulates behind with the exoccipital, in front 

 with the supraoccipital {s.o) and parietal {-pa), and externally with the pterotic {pt.o), 

 while from its inner or cranial surface an irregular snag-like process is given off, which 

 articulates with a similar process of the opisthotic (figs. 12 and 13, op.o'^). The arch 

 of the posterior semicircular canal runs through this bone, which I have no hesitation in 

 identifying as the epiotic, in spite of its unusual size. 



The supraoccipital (s.o). — Continuing forwards, the epiotic is a median shield-shaped 

 bone, forming the central part of the skull-roof, and bounded in front by the 

 cartilaginous tegmen cranii. It is a cartilage bone, and is clearly the supraoccipital 

 displaced from its proper connection with the exoccipitals by the abnormal development 

 of the epiotics. Externally it articulates with the parietals (pa). Its middle portion is 

 very thick, and is covered internally with unusually thick periosteum (fig. 11). 



The opistliotics {op.o) are of very irregular form, and are perhaps the most remark- 

 ably modified bones in the skull. They are united with one another in the middle 

 ventral line forming the anterior two thirds of the basis cranii, and thus coming to 

 articulate posteriorly with the basi- and exoccipitals. Each opisthotic sends ofl'a large, 

 irregular, ascending process (figs. 12 and 13 op.o '), which articulates with a similar 

 process of the epiotic ; a forward process [op.o ^), which articulates with the alisphenoid 

 {als) and unites with its fellow of the opposite side ^ ; a posterior process [op.o ^) articu- 

 lating with the basi- and exoccipitals, and uniting with its fellow in the middle line ; 

 and a thin, laminar, descending process {op.o *), which articulates with the basioccipital 

 behind and with the parasphenoid below, and, with its fellow, forms the anterior or 

 double portion of the subcranial crest. The anterior process is perforated by a small 

 aperture ( V), which I am disposed to think, from examining a very badly preserved 

 specimen of Trachypteris altivelis, transmits the third division of the fifth nerve. With 

 the exception of the descending process, the opisthotic does not appear on the outer 

 surface of the skull, the otic region being formed externally largely of unossified 

 cartilage (figs. 7 and 9). 



Ihe prootics [pr.o). — These are small bones, consisting of a considerable groundwork 



' Stannius, ' Zootomie der Fische,' p. 56. 



^ As the skull is bisected, I cannot state positively whether the exoccipitals, opisthotics, and orbitosphenoids 

 are actually ankylosed with one another, or only united by suture ; I think, however, that there is actual 

 ankylosis in the case of the two former. 



