253] PROTEOCEPHALIDAE—LA RUE 253 



hea4 are reproduced (Figs. 33, 34). Monticelli's (1899) description of 

 the head reads: "Come si vede, il capo (Monticelli's figures of the 

 head are reproduced in figures 123, 124) e abbastanza grande e ben 

 distinto dal coUo che anteriormente si slarga di poco come pei sostenerlo : 

 esso ha I'aspetto grossolano di un porno da bastone e, come si rileva 

 dalla figura, e rigonfio e massiccio, tetragonale, a forma di piramide 

 tronca. Anteriormente ristretto, cupuliforme, terminato nel mezzo da- 

 un indistinto cocuzzoletto apicale conoide ; posteriormente slargato e v 

 troncato sporge oltre il coUo per i suoi spessi margini rigonfi e presenta, 

 nel mezzo di ciascuno dei quattro lati, una insenatura molto accentuata. ' ' 

 The head is always tetragonal and somewhat pyramidal. This 

 shape may be varied to some extent by the states of contraction. A 

 head which is relaxed and whose suckers are expanded may have a 

 lobate appearance (Fig. 13). Here deep grooves may be seen between 

 the suckers. In greatly contracted heads (Fig. 12) the suckers are 

 drawn closely together, the furrows between the suckers are almost 

 obliterated and the suckers themselves are reduced in size. In such heads 

 the suckers are seen to be directed forward while in the relaxed head 

 the suckers are directed outward. These figures should be compared- 

 with the drawings of heads by Smith (1908) and Monticelli (1899), 

 which have been reproduced here (Figs. 33, 34 and Figs. 123, 124). 

 These figures illustrate the various states of contraction which one meets 

 in a study of considerable material. The dimensions of the head are: 

 breadth 0.935-1.30 mm. in the writer's material, 1.75 mm. (Baird), 

 thickness nearly equals breadth, length about 0.68-0.90 mm. The apex 

 of the head is slightly elevated, rarely flat. There is no rostellum and 

 no spines. 



From toto preparations no trace of a fifth sucker could be found 

 but in a series of transverse sections of the head a small sunken fifth 

 sucker was found which had degenerated until it had lost its radial 

 musculature (Fig. 42). The sucker still possessed its opening to the 

 exterior which measured 0.037 by 0.026 mm. and this opening led into 

 a cavity but a little more than 0.03 mm. deep. The sucker measured 

 about 0.048 by 0.067 mm. on its transverse axes. From the tip it ex- 

 tended about 0.06 mm. into the head. This tissue was in bad condition. 

 It took the stain poorly. Its nuclei were not visible either in the sucker 

 or in the other parts of the head. Cell outlines were indistinct or lack- 

 ing. The musculature of the sucker was irregular or perhaps no longer 

 visible. There were however certain fibers which seemed to be muscles. 

 A basement membrane was clearly visible and surrounding this on the 

 outside were circular muscle fibers which could be readily distinguished 

 by focussing on successive layers of tissue. The sucker cavity was filled 



