324 EDWARD S. MORSE ON 



lower shell were quite unlike anything found in D. lamellosa. A narrow median line is 

 seen, upon each side of which three eccentric lines spring from a nucleus, the larger curve 

 of the eccentric being directed backward ; in front of this a curved line crosses the 

 median line and beyond this an indication of the continuation of the median line appears. 

 This central line is the only feature seen in D. lamellosa. 



COECAL TUBULES. 



A very marked characteristic of the Testicardiue group is the presence, in the shell of 

 many of them, of tubules with which the pallium has an organic connection ; even the parts 

 surrounding the peduncular foramen are supplied with them. Dr. Beecher ('92) shows 

 that this statement is especially true with respect to the Telotremata, for in this order the 

 deltidial covering, consisting of the deltidial plates, is secreted by the mantle border, and 

 in punctate forms the deltidial plates are likewise punctate. On the contrary, the delti- 

 dial covering, or deltidium, in the Protremata ( Thecidium, Strophomena, etc.) is prima- 

 rily secreted by 'the peduncle and as such is always impunctate, even in the most highly 

 punctate species, although the obliteration of the peduncle may result in a secondary 

 punctate deposit secreted by the mantle within the delthyrium. Precisely what their 

 functions are is still problematical. Various suppositions have been made by those who 

 have studied these structures microscopically. It has been suggested that they might be 

 respiratory, again it has been suggested that they were instrumental in conveying growth 

 elements to the shell. Kowalevski ('74) is of this opinion; Schulgin ('84) is inclined 

 to agree with him, but thinks they may have a respiratory function as well. From the 

 glairy nature of the shells of the smaller Llncjulidae one might suppose that in some 

 way they functioned as mucous tubes, but in these forms only the barest traces of 

 their existence have been detected, and in no brachiopod have these pores been seen to 

 open externally, at least in the adult form. It was long ago established that coecal 

 prolongations of the pallium project into these tubular perforations of the shell. In all 

 cases, at least in the adult, a delicate periosteum covers the shell, and consequently the 

 tubules do not communicate with the exterior. Briefly stated, these processes may be 

 said to be aborted in the errantiau forms and to be, with few exceptions, a marked 

 characteristic of those forms which are attached by a peduncle, or of those forms in which 

 the lower shell is welded to the substance upon which it rests. In Crania, which has no 

 peduncle, and which is immovably attached by the lower shell, the tubules are numer- 

 ously branched towards the exterior of the shell. Joubin ('86) finds that in Crania the 

 tubules are branched in the dorsal shell only, and that they do not occur in the areas of 



