FOSSIL AND RECENT 89 



(1965 : 34) also stated that P. propterus is genetically distinct from the English 

 species. In support of this statement Bardack refers to five points of dissimilarity 

 between the English and Solenhofen species : the proportions of the body ; structure 

 of the neurocranium ; form of the dentition ; number of vertebrae and the relative 

 positions of dorsal and anal fins. I have been unable to examine ' typical ' specimens 

 of P. propterus but the description and figures given by Nybelin (1964) are sufficient 

 to invalidate the above points. Firstly, the body proportions are very similar in 

 both the English and Solenhofen species and differ somewhat from Thrissops and 

 Allothrissops. The head length of all Pachythrissops species is contained approxi- 

 mately four times in the standard length and thus differs from the contemporaneous 

 ' chirocentrids ' in which the head length never exceeds one-fifth of the standard 

 length. In the body depth there is virtually no difference between P. laevis and P. 

 propterus whereas P. vectensis appears slightly more slender. The relative depth of 

 the cranium is also similar in all the species of Pachythrissops. 



The unspecified differences in neurocranial structure mentioned by Bardack 

 (1965 : 34) are equally difficult to understand. The degree of ' similarity or differ- 

 ence ' in neurocranial structure may be taken at various levels depending on how 

 well the neurocrania are known. None of the Pachythrissops species is well known 

 and in consequence comparison must be at a superficial level. In as much as the 

 English and Solenhofen species show shallow neurocrania with a flat roof and a 

 straight, edentulous parasphenoid, with a weak development of the lateral ethmoid 

 region, there is a degree of resemblance. Such resemblances are of a primitive nature 

 with the exception of the weakly developed lateral ethmoid, and are found in other 

 contemporaneous teleosts. 



The number of vertebrae is similar in P. laevis and P. propterus ; the count for 

 P. vectensis is unknown. P. laevis shows 53-56 vertebrae of which approximately 

 22 are caudal. According to Nybelin (1964 : 34) P. propterus has 50-54 vertebrae 

 (24-25 caudal). 



The differences in the relative positions of dorsal and anal fins (Bardack 1965) 

 appear to be erroneous. In both P. laevis and P. propterus the fins are posteriorly 

 placed and the dorsal fin originates slightly anterior to the anal fin. The slight 

 difference in the position of the dorsal fin in these two species is of minor importance 

 since it is often subject to specific and even ontogenetic variation. 



Further points of similarity between the English and Solenhofen species are seen 

 in the jaws. The premaxilla forms nearly one-third of the oral margin of the jaw. 

 The dentary exhibits an inflected ventral margin and the oral margin rises steeply to 

 produce an elongate coronoid process (this feature is seen in several other contem- 

 poraneous teleosts, e.g. Allothrissops and Leptolepis). The dentition is represented 

 by a band of small teeth upon the premaxilla, maxilla and dentary. Those of the 

 dentary become slightly larger at the symphysis. Saint-Seine (1949) attributed 

 large toothed forms to Pachythrissops and concluded that this genus was carnivorous. 

 It is probable that the specimens he attributed to Pachythrissops were in fact 

 Thrissops formosus. 



The above brief notes suggest that there is, at present, no basis for separating 

 Pachythrissops propterus from the English species. 



