22 Percy E. Raymond — The. Pygidntm of the Trilohite. 



in throwing further Hght upon the problem of the " Brachiopod 

 Beds ". Little or nothing is yet known regarding the transition 

 from these " knoll-reef " deposits into the standard limestones 

 of the district, while their rich fauna affords a great field for dis- 

 criminating study. 



The Pygidium of the Trilohite. 



By Percy E. Raymond, Ph.D., Harvard University, Cambridge, 

 Massachusetts, U.S.A. 



A LTHOUGH the process of development of the pygidium in the 

 -^-^ trilohite has been known since the time of Barrande, its 

 significance does not appear to have been appreciated, and much of 

 the modern speculation on the classification of trilobites is based 

 upon the erroneous idea that those species whose form is most 

 annelidan are most primitive. As a matter of fact, the study of 

 both morphology and ontogeny shows that in this group the large 

 pygidium is 23rimitive and the small one sj^ecialized. 



The growing jDoint in trilobites is, as in all other arthropods, 

 immediately in front of the anal segment. This was proved by 

 Beecher from a study of the appendages of Triarthrus, and is con- 

 firmed by facts more recently afforded by specimens of Neolenus and 

 Cryptolithus having the appendages attached. New segments must, 

 therefore, be introduced during moults, and each is pushed forward 

 through the pygidium by those which are added later. While these 

 segments are, as thus introduced, potentially free, they do not 

 actually become so until they are pushed off from the anterior end of 

 the pygidium. The thorax, therefore, grows through the degeneration 

 of the pygidium. This process can be seen in the ontogeny of all 

 species where any considerable number of growth -stages are known, 

 but can be studied in greater detail in Barrande's classic descrip- 

 tions of Sao hirsuta and Dahnanites so:iialis. In all cases it is found 

 that the pygidium of the protaspis is ^proportionally larger than at 

 any later stage in the life-history, and this is equally true of an 

 elongate Mesonacid or of a Phacopid such as Dahnanites. If the 

 proportional length of the pygidium of Sao at various stages be 

 examined, it will be found that in the very smallest protaspis it is 

 about 30 per cent of the whole, at a length of 1 mm., while there 

 are still no free segments, the animal is j^ractically isopygous, and 

 from this stage to a length of 7 mm. the projDortional length steadily 

 drops to the 7 or 8 per cent which remains in the adult. All 

 trilobites, whether the adult has a large or a small pygidium, 

 show this same process, and Troedsson has recently described a 

 particularly interesting case. The j^ygidium of Dahnanites eucentrus 

 (Angelin), when I" 28 mm. long, has eight pairs of pleural ribs, 

 indicating the 230ssession at that stage of two more segments than 

 the adult. ^ This case is of the more importance on account of the 



1 Lunds Univ. Arsskr., n.f. avd. 2, bd. 15, nr. 3, 1918, pp. 57-67, pi. i, fig. 23. 



