1846.] MORRIS ON TEREBRATULA. 385 



which only bounds the lower or anterior part of it or leaves it open 

 below, this area varying very much in extent, form and direction, 

 sometimes the area forming but a very limited portion of the enclo- 

 sure of the foramen, at others enclosing one-third or one-half of it, 

 but never entirely surrounding it (see fig. 1 and 2). Within this area 

 the deltidium is more or less developed, and when the foramen is en- 

 tire the deltidium generally fills up the whole space of the area between 

 the anterior part of the foramen and the umbo of the ventral valve 

 {T» australis, &c.). In some cases the deltidium extends nearly 

 across the area, leaving a small fissure in the centre, so that the 

 foramen is open below ( T. dorsatd) ; in others it occupies but a 

 small portion on each side of the base of the area, and in this case 

 the opening is large, being formed by the foramen and the unclosed 

 portion of the deltidial area together, and only bounded by the ven- 

 tral valve (71 rubra, T. truncata, T.detruncata)^. All the species 

 having these characters, whether plicated, costated or smooth, have 

 a punctated structure, and are generally longer than wide. The 

 species constituting this section, and which may be considered as 

 true Terebratulae, are very various in external form and character, 

 and may be subdivided into four or five minor groups ; they are 

 readily distinguished from those of the next section by the absence 

 of the acute beak, and by the fact that the deltidial area never sur- 

 rounds the foramen. All the recent species at present known (M'ith 

 two exceptions) belong to this division, but of the fossil forms be- 

 longing to it there are very few simple plicated species {PlicoscBy 

 Von Buch), while of the DichotomcB, species with bifurcating costaB 

 or striae, there are four divisions differing in the greater or less de- 

 velopment of the deltidium. In the first of these, Costatce {T. car- 

 dium, Lam.), the beak is slightly produced and the foramen is entire. 

 In the second, Rostratce (T. li/ra, Sow.), the dorsal beak is much 

 produced, the foramen terminal, and the deltidium extending from 

 below it to the ventral valve. In the third, Striates {T, striatula, 

 Mant., T. caput- serpentis), the deltidium is only partially developed 

 towards the base, and the foramen is consequently not perfectly en- 

 closed. In the fourth, Expansce. ( T. truncata. Lam. &c.), the del- 

 tidium is also only partially formed, and the opening is generally 

 large, combined with a straight cardinal area. In the Loi'icatcB 

 ( T. Menardi, Lam.) (see fig. 2) the shell is wider than long, and 

 the deltidium separated or rarely continuous, so as to produce an 

 entire foramen ; the cardinal area is large and straight. In the 

 CinctcB ( T. quadrifida. Lam.) the beak is slightly curved, the fora- 

 men is small and entire, as is also the deltidium, which apparently 

 surrounds about one-half of the opening. In the small size of the 

 foramen the Cinctae are well-contrasted with some of tlie other 

 smooth species belonging to the JugatcB and Carinatce. In the Ju- 



is placed below the beak in all the species of Tcrcbratula, and which is more or 

 less filled up by the two pieces of shell called the deltidium by Von Buch, and 

 which separates the foramen from the hinge line of the ventral valve. 



* Vide * Thesaurus Conchyliorum,' part 7, by G. H. Sowcrby, for figurcij of these 

 recent species. 



