108 



THE GEOLOGIST. 



— regards these as being due to the occlusor (his valvular") muscle?; 

 for it is highly probable that these smaller scars in advance of the 

 larger ones were produced by a portion of the occlusors themselves.'" 



Immediately under, but outside of, these there exists two deep, 

 longitudinally striated, subquadrate impressions, which are in all 

 probability due to divaricator (= cardinal) muscles, but which 

 have been referred by Professor King to pedicle muscles, t In vain 

 hitherto have I sought for impressions referable to adjustor muscles ; 

 but, as no peduncle existed in the Productidce, such muscle might be 

 supposed not to have existed ; however, as the valves of some of these 

 forms possess no teeth nor sockets, and, therefore, are not strongly 

 articulated, as in the Terebratulidce, it is not impossible that the 

 adjusters may have been so arranged in Producta and Aidosteges as to 

 keep the valves adjusted to each other, and that they have thus acted 

 as a substitute for a hinge, somewhat in the manner Mr. Hancock 

 has supposed to be the case in Liagula.X 



* Mr. Hancock informs me that the occlusors are undoubtedly formed of two 

 elements, the anterior and posterior, and that we should not therefore be snr- 

 |)rised to find indications of the two component parts in the ventral as well as in 

 the dorsal valve. In Lingula the anterior and posterior occlusors are distinct, 

 liaving four points of attachment in each valve. 



f- Prof King has figm-ed in his monograph of Enoiish Permian Fossils (PI. XIX. 

 fig. 2) what he terms vascular markings in connexion with these large muscular 

 scars, and which seem to form part of the impression. ^Ir. Hancock appears dis- 

 posed to consider the wdiole to be the scar of one muscle, and that there is nothing 

 extraordinary in this, as it h-equently happens tliat the same thmg may be seen 

 in the rnionidai and other Conchifera. 



t Although the case in question may not apply directly to Producta, it will be 

 as well to mention that Mr. Hancock has found in Lingida three pahs of adjustors, 

 apparently for the puipose of keeping the valves op])osed to each otlier and of 

 holding them adjusted. In this respect, they appear well calculated to compensate 

 for the entire absence of hinge or teeth. He explains this in the following words : 

 — " The external or ventral pair having their anterior extremities attached to the 

 ventral valve— wdiich, as it is fixed to the peduncle, is that from wliich all nniscles 

 act — and their posterior ends to the dorsal, it is evident that they will prevent 

 the latter from being forced backAvards ; while the posterior adjustors having their 

 terminations united to the ventral or fixed valve. n\v\ their anterior portion to the 

 dorsal, they will act in the contrary direction, and guard against the pressure for- 

 ward ; they will also at the same time prevent any lateral displacement of the 

 valves, as their diagonal position will enable them to act transversely, as well as 

 longitudinaUy. The external and central adjustors will, on account of their 

 oblique arrangement, exert a similar double influence in front." See JNIr. 

 Hancock's admirable memoir on the anatomy of the Brachiopoda, published in 

 the " Transactions of the Royal Society" fur 1858. 



^Ir. Howse remarks, in his paper, published in the Annals of Natural History 

 (1857), that, Avhen the cardinal process of Producta is in sitn, it fills nearly the 

 whole of the umbonal cavity of the ventral valve, and may thus assist in keeping 

 the valves in position. It is possible, however, that future researches by the aid 

 of better preserved specimens, may enable us to ihscover some traces of adjustor 

 muscles. 



