74 GENETIC STUDIES ON A CAVY SPECIES CROSS. 



16. MISCELLANEOUS MORPHOLOGICAL CHARACTERS. 

 THE INTERPARIETAL BONE. 



An interparietal bone occurs in young guinea-pigs, but after a few 

 weeks it generally becomes fused with the parietals and can not be 

 detected. We do not know whether it ever occurs in the adult wild. 

 Table 70 shows its occurrence in the wild, tame, and hybrid guinea-pigs 

 which were available for study. Figures 13, 18, 19, and 33 show its 

 form, usually a very distinct triangular bone. Its occurrence in guinea- 

 pigs is infrequent. It occurred in 9 out of 141 guinea-pigs, or 6.4 

 per cent. None of these guinea-pigs were used in matings with the 

 wild or hybrids. Among the | wild it was found in two cases, or 15.4 

 per cent. These two cases were a brother and sister, but none of the 

 subsequent hybrids showing an interparietal bone were descendants 

 of these two. 



The interparietal was present in 15 out of 46 I wild hybrids, or 32.6 

 per cent. Eight of the 9 J wild females showing it were mated with the 

 guinea-pig, and 5 of them had some offspring which also showed it. 

 But other J wild females had offspring which showed the same anomaly. 

 In other words, some of the 23 j wild hybrids showing an interparietal 

 bone were descended from females which had it, while others were 

 descended from females showing absolutely no trace of it. The inter- 

 parietal seemed to be most frequent (32.6 per cent) in the J wild, and 

 when these were mated to guinea-pigs the | wild showed it in 18.4 

 per cent. One would expect it to decrease in frequency, for continually 

 mating back to the guinea-pig should eventually establish the zygotic 

 constitution of guinea-pigs in most dilute hybrids, and thus reduce the 

 frequency of an interparietal bone to that of a race of guinea-pigs. 



THE SHAPE OF THE SKULLS. 



The skull of the wild C. rufescens is specifically much more pointed 

 than that of C. porcellus (see figs. 10, 11, 15, 16, and 31). In crossing 

 these two species, the Fi, or § wild, was an apparent blend (see figs. 

 12, 17, and 32). Crossing the Fi generation back to the guinea-pig 

 gave some forms just like the guinea-pig, although most of them showed 

 traces of the wild influence (see figs. 13, 18, and 33) . The next back- 

 cross, giving the f wild or F3 generation, were in general similar to the 

 guinea-pig, but possibly showed a little wider range. 



To ascertain the magnitude of pointedness or triangularity of a skull 

 is difficult. If one takes the ratio of the greatest width of a skull to 

 the width at the laterocaudal margin of the infraorbital foramen, one 

 obtains an idea of the triangularity; but the quotients thus obtained 

 can not be regarded as more than approximations. Measurements 9 

 and 11 were the widths of the skulls at these two levels. Dividing 

 measurement 9 by measurement 11 gives an index of the triangularity; 



